Experimental investigation of heat transfer coefficients ...

Energy and Buildings 82 (2014) 211–221

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Experimental investigation of heat transfer coefficients between hydronic radiant heated wall and room Aliihsan Koca ∗ , Zafer Gemici, Yalcin Topacoglu, Gursel Cetin, Rusen Can Acet, Baris Burak Kanbur Mir Arastirma ve Gelistirme A.S., 34220 Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 21 February 2014 Received in revised form 24 June 2014 Accepted 25 June 2014 Available online 5 July 2014 Keywords: Heat transfer coefficient Radiant wall heating system Thermal comfort Convection Radiation

a b s t r a c t The reason radiant heating systems are highly preferred in recently designed or a modernized construction is due to their energy efficiency and low exergy destruction. Radiant heating systems are different from typical HVAC systems because they heat surfaces rather than air and can save large amounts of energy while providing higher levels of comfort. The fundamental design parameters for a radiant heating system are heat transfer coefficients such as radiation, convection and total heat transfer coefficients. In this study, the values of radiant, convective and total heat transfer coefficients were calculated based on the experimental results that were taken from the climatic test chamber. This article presents the results based on an experimental research on heated radiant hydronic wall panels mounted in the climatic test chamber. The aim was to estimate the values of heat transfer coefficients for different location configurations of radiant wall heating panels experimentally. This experimental study was conducted under the location configurations of 3 different wall panel arrangements for 7 water flow temperatures ranging from 30 ◦ C to 42 ◦ C. It was noticed that the amount of radiation heat transfer increased when the crosswise area of the window surface was heated and simultaneously the heat transfer capability of radiation rose by 70% of the total heat transfer. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Since number of environmental problems, one of which is global warming, has increased and its magnitude has started to threaten peoples’ lives, the efficient usage of energy has become of the most important issues. In many fields, people are trying to lower the energy consumption and use energy in more efficient ways. The energy usage in buildings is one of the biggest portions of the total energy demand in the world, and that is considered to be 40%. The main reason for this high energy consumption rate is HVAC systems, which are mounted on buildings for cooling and heating applications. Implementing positive change and that is to reduce the energy consumption rate in this field requires a great amount of research. Moreover, technology has been improving quite fast recently and many papers have been released [1–11].

∗ Corresponding author at: Yildiz Teknokent A1 Blok, Mir Ar Ge Sirketi, 34220, Esenler, Istanbul, Turkey. Tel.: +90 549 7483609; fax: +902124837073. E-mail address: [email protected] (A. Koca). http://dx.doi.org/10.1016/j.enbuild.2014.06.045 0378-7788/© 2014 Elsevier B.V. All rights reserved.

In terms of the aforementioned facts, radiant heating-cooling systems, which are quite convenient alternatives for the traditional HVAC systems, meet the efficiency requirements by producing more comfortable environments. They reduce energy consumption [12–21] because of low-temperature heating and cooling operations. The radiant heating and cooling system consists of large radiant surfaces installed on room walls, floors or ceilings. A conditioned surface is called as a radiant panel if 50% or more of the designed heat transfer on the temperature-controlled surface takes place by thermal radiation [22]. Thus, it is feasible to use lower supply temperatures for heating and higher supply temperatures for cooling. This factor improves energy efficiency and at the same time reduces energy consumption. In addition, they could easily be combined with alternative low exergy sources such as geothermal energy, groundwater, heat pumps and recovered waste heat [23–26]. The indispensable parameters used in radiant heating and cooling systems are heat transfer coefficients. The determination of heat transfer coefficients for the radiant panels is fundamental to the design procedure of radiant heating-cooling systems, CFD analysis, thermal comfort studies, building information and modeling

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A. Koca et al. / Energy and Buildings 82 (2014) 211–221

Nomenclature A Ta Top Ts Tw Tref AUST htot hr hr qtot qc qr U F

area (m2 ) air temperature (◦ C) operative temperature (◦ C) surface temperature (◦ C) Water temperature (◦ C) reference temperature (◦ C) Area weighted average temperature (◦ C) Total heat transfer coefficient (W m−2 K−1 ) radiation heat transfer coefficient (W m−2 K−1 ) convective heat transfer coefficient (W m−2 K−1 ) total heat flux (W m−2 ) convective heat flux (W m−2 ) radiation heat flux (W m−2 ) coefficient of thermal transmittance of surfaces (W m−2 K−1 ) view factor

(BIM). The calculation for radiant wall systems as in (Eq. [1]), which is given in EN 1264-5 [27] and EN 15377 [28], is often used in engineering processes for radiant heating and cooling. qtotal = 8.(Top − Ts )

(1)

To design the radiant system, the radiant heat transfer coefficient (hr ) and convective heat transfer coefficient (hc ) values are necessary to estimate the heating and cooling capacity of a radiant system. In the literature, several values and calculations for heat transfer coefficients are given by Khalifa [29–31], Awbi et al. [32–34], Karadag et al. [35–37], Min et al. [38]. The average convective heat transfer coefficient in the literature for a heated floor or a cooled ceiling configuration, are between 2.8 and 4.8 W m−2 K−1 . And the radiant heat transfer coefficient is between 5.4 and 6.2 W m−2 K−1 [28,39,40]. Furthermore, total heat transfer coefficients range from 7.8 to 9.3 resulting in an average value of 8.5 W m−2 K−1 [29–39]. So far, a great deal of research on thermal properties and the heat transfer coefficients for radiant heating and cooling systems have been conducted. Andres-Chicote et al. [41] had done an experimental study on the cooling capacity of a radiant cooled ceiling system. A series of experimental tests about hydronic heating and cooling radiant ceiling panels were performed by Fonseca Diaz [42] Tian et al. conducted research on the actual cooling performance of ceiling panels [43]. Dongliang et al. focused on the operating characteristics of lightweight radiant floor heating systems by analyzing experimentally and numerically [44]. Okamoto et al. developed a calculation method for estimating heat fluxes from ceiling radiant panels for different insulation materials and supply water temperatures in heating and cooling cases [25]. Causone et al. evaluated the heat transfer coefficients between a radiant ceiling and a room in typical conditions of occupancy in an office or residential building [45]. He indicated that, for the heated and cooled ceiling the radiant heat transfer coefficient can be considered constant at 5.6 W m−2 K−1 . The data is very close to the ones calculated by Olesen et al. [39] Furthermore the total heat transfer coefficient for cooled ceiling has an average value of about 13.2 W m−2 K−1 . These values are significantly higher than the ones typically shown in the literature (∼11 W m−2 K−1 ) [40]. Cholewa [46] proposed the total heat transfer coefficient for heated floor in the range of (8.5–11.1 W m−2 K−1 ). Furthermore he proposed using the value of radiant heat transfer coefficient (hr ) of 5.6 W m−2 for heated radiant floor, as recommended by Causone et al. [45].

Myhren et al. [24] did research to find out how different heating systems and their locations affect the indoor climate. Tye-Gingras et al. carried out a numerical study on radiant heating panels which were modeled at different locations in the room and the effects of these applications on thermal comfort were discussed by taking the results numerically from the CFD analysis [47]. In the present study, heat transfer coefficients were calculated for different wall panel location configurations by using a real size test chamber. Most of the submitted studies have been encouraging for ceiling and floor heating-cooling systems. However, in this study the heat transfer coefficients of wall mounted radiant heating systems were investigated. The reason a real size test chamber was used in this study was to obtain the most realistic results and to compare them with the heat transfer coefficients which were calculated before by using some prior standards. One of the goals of this study is to discuss to what extent the results obtained by these standards coincide with real life conditions. The main aim of this research is to provide further experimental information for a better understanding of heat transfer phenomena from radiant wall heating surfaces and to clarify the vague conceptions in heating operations. For this reason, the heat transfer coefficients, which were investigated under different thermal conditions and heating operations in the test chamber, were fulfilled by advanced test chamber measuring devices. The outside conditions of the test chamber were adjusted to a certain temperature. Heating tests were done for three different location configurations which are the west wall (case-1), the north (case-2) wall and the both combined (case-3). The experimental facility for measuring the system’s heating output was provided. Natural convection conditions and a wide range of tolerable temperatures were also considered during the investigation. The heat transfer correlations were studied and the amount of radiation and convection heat transfer was calculated for all different configurations. Then, the findings were analyzed and compared with those found in the literature.

2. The experimental setup 2.1. The arrangement of the test chamber The climatic test chamber was constructed to get realistic test results for different heating applications under different climatic conditions. The climatic chamber used during the tests was built in such a way as to reproduce the possible structure and characteristics of a real size room which is presented in Fig. 1 as accurately as possible. This chamber, which is composed of 5 volumes: ceiling (volume-1), floor (volume-4), exterior (volume-2), inner volume (volume-3) and studied volume (volume-5), encloses the test chamber, which is characterized by a floor area of 24 m2 (6.00 m × 4.00 m) and an internal height of 3.00 m. The wall types are chosen as the sandwich type with polyurethane insulation between two layers made out of sheet steel which has engagement and locking mechanism to increase the strength. The insulation thicknesses and the coefficient of thermal transmittance of walls for different zones were determined according to Turkish Standard (TS) item 825 (thermal insulation requirements for buildings), presented in Table 1 [48]. The window, (115 cm × 160 cm) which overlooks the zone-2, is double-glazed and the door (82 cm × 204 cm) is single glazed with U-value of 2.20 W m−2 K−1 and 2.60 W m−2 K−1 , respectively. Temperature, humidity and air velocity range of the composed 4 volumes are presented in Table 2.


213

Fig. 1. General view of the climatic test room.

2.2. The radiant panel The radiant panel (Fig. 2) that was designed for this study consists of four layers which are drywall, aluminum foil, heating pipe serpentine and insulation; from inner to outer layers. The serpentine is imprinted on the drywall. The thickness of the drywall layer is 15 mm while the panel insulation thickness is 30 mm. There is also an aluminum foil layer of 0.3 mm thickness between the drywall and the insulation layer which wraps the slot that the serpentine is imprinted. The serpentine has cross-linked polyethylene (PEX) pipes with a 10.1 mm diameter and 150 mm pipe spacing. Expanded polystyrene (EPS) is used as an insulation material which

Table 1 Coefficient of thermal transmittance of surfaces.

Fig. 2. Sample of radiant heating wall panel.

Surfaces

U (W/m2 K)

Ceiling (volume-1) Floor (volume-4) North wall West wall East wall South wall

0.3 0.4 0.4 0.4 0.8 0.8

has a coefficient of thermal transmittance value of 0.035 W m−1 K−1 (at 10 ◦ C). The test chamber was set up with the installation of 7 radiant wall panels (The dimensions of the wall panel are 2.2 m × 1 m) which were mounted on the entire northward and westward walls. Two of them were located on the northward wall at both sides of the

Table 2 Controlled parameters in volumes. Parameter

Volume-1

Volume-4

Volume-2

Volume-3

Temperature Humidity Air velocity

−10 ◦ C/+40 ◦ C n/a n/a

+10 ◦ C/+26 ◦ C n/a n/a

−10 ◦ C/+40 ◦ C %35/%85 RH 0.5–5 m/s

+10 ◦ C/+26 ◦ C n/a n/a

214


panels, the water comes to the four-way valve again through the return line and it is mixed with the water that comes from the tank if needed (to adjust the required temperature of the fluid).

2.4. The measurement equipment The indoor air temperatures of the test chamber were measured by using K-type thermocouples (chromel–alumel), which are located vertically 10 cm, 110 cm, 170 cm and 250 cm above the floor and placed in two different positions symmetrically with respect to the center of the room. In this way, a more uniform and accurate temperature distribution was achieved with the measurements taken. The relative humidity was measured at two points which are at the same location with thermocouples. The indoor humidity was measured by using a sensor with a sensitivity of ±3.5%. The temperature of each unheated surfaces were measured from the middle point of the walls by type K thermocouples. Four thermocouples were located on the west facade wall on the same line (the interior wall, the insulation layer, the interior surface of the facade room and the exterior surface of the facade room) at 1.5 m. The surface temperature for heated surfaces was detected by the average values of the thermal camera results as shown in Fig. 5. Furthermore, a thermocouple was placed on the window surface. The water flow rate at the inlet of each panel group was measured by three flow meters. The operative temperature was measured by thermal comfort measurement equipment which was located at the center of the chamber as shown in Fig. 6. The data from the sensors were transmitted to the respective signal conversion panels on PXI and stored in a personal computer. The data was recorded at intervals of 1 min. The LabVIEW software, which allowed the graphs of the data values to be viewed in real time, was used to process the data. The software also provides the opportunity to set parameters and automation.

2.5. The experimental method Fig. 3. Legend of the radiant heating wall panel arrangements (a) Case 1, (b) Case 2, (c) Case 3.

window and five of them were located on the westward wall. Fig. 3 shows the different arrangements of the heated panels: In case 1, two panels were mounted on the northward wall; in case 2, five panels were mounted on the westward wall and in case 3, seven panels were mounted on both northward and westward walls. 2.3. The hydraulic system of the test room The temperature of the water pumped to the panels may differ according to the heating season for different climatic conditions. A water conditioning system was used in the test chamber. With this system, water is circulated to the panels for all the 3 case studies in order to get more reliable results at the end of this research. As shown in Fig. 4, the inlet water, which has access to the system through the tank (the water temperature is obtained by means of a chiller for the cooling case and by means of electric resistances for the heating case), firstly comes to a four-way valve. Here, the four-way valve provides a mixture through a return pipeline. The mixed water temperature leaves the four-way valve so as to be equal to the water inlet temperature and enters the pump to supply the needed pressure. After the pump, the water comes to a threeway valve. The purpose of the three-way valve is to return excess of the fluid back so as to remain equal to the flow rate of the water when the water which comes from the pump has a higher flow rate than required. Then, the fluid passes through the flow meter, where the volumetric flow rate is measured. After finishing the cycle in the

The most reliable analyses are those based on measurements performed in full scale test chambers. In this paper, three cases were studied experimentally. For all cases, the water flow rate and the environmental conditions were fixed at certain values. The object of this study was to calculate the heat transfer coefficients between the radiant wall panels and the chamber air. According to the EN 14240 standard [49], the measurements were carried out under steady conditions for at least three different temperature differences between the chamber air temperature and the mean heating water temperature. Within the scope of the experimental studies, seven different water supply temperatures were tested for each case. The emissivity of the chamber surfaces and radiant floor surfaces, which were used for radiant heat transfer calculations, was estimated by the use of an infrared thermal imaging camera and calibrated thermocouples. The surface temperature was measured by using temperature sensors. Then, the surface emissivity was changed in the pyrometer setup in order to get the same temperature of the analyzed surface as obtained before by the use of the temperature sensors [39]. All the air temperature sensors and the surface temperature sensors (K-type thermocouples) were calibrated and their accuracy was equal to 0.1 K. All the measured values by the use of the calibrated sensors were archived with the time interval which is equal to 1 min. But the values of the heat transfer coefficients were estimated after the analyzed system reached a steady state. It means that the analyzed system in a steady state was characterized by physical properties that were unchanging in time.


215

Fig. 4. Hydraulic system.

Fig. 5. Temperature distribution on the panel surface with thermal camera measurement. Table 3 Uncertainty analysis results. Sup. water temp. (◦ C)

Q (w)

Q (%w)

qr (w)

qr (%w)

qc (w)

qc (%w)

qt (w)

qt (%w)

hr (w)

hr (%w)

hc (w)

hc (%w)

ht (w)

ht (%w)

Case-1

30 32 34 36 38 40 42

2.25 2.46 2.67 2.89 3.08 3.24 3.43

1.02 1.01 1.01 1.01 1.01 1.01 1.01

0.085 0.085 0.085 0.086 0.086 0.087 0.087

0.30 0.27 0.25 0.23 0.22 0.21 0.19

0.52 0.57 0.61 0.66 0.71 0.74 0.79

3.21 3.18 3.14 3.13 3.04 3.08 3.07

0.12 0.13 0.14 0.15 0.16 0.17 0.18

0.266 0.263 0.260 0.258 0.255 0.254 0.253

0.086 0.079 0.072 0.067 0.062 0.060 0.057

1.60 1.45 1.33 1.23 1.14 1.09 1.03

0.110 0.108 0.106 0.106 0.104 0.105 0.105

3.58 3.49 3.41 3.36 3.25 3.27 3.25

0.30 0.28 0.27 0.26 0.25 0.25 0.24

3.51 3.32 3.13 2.98 2.81 2.80 2.74

Case-2

30 32 34 36 38 40 42

4.07 4.41 4.66 4.99 5.35 5.68 6.07

1.01 1.01 1.01 1.01 1.01 1.01 1.01

0.025 0.025 0.025 0.026 0.026 0.026 0.026

0.11 0.08 0.07 0.07 0.06 0.06 0.06

0.37 0.40 0.42 0.45 0.49 0.52 0.55

3.92 3.91 3.88 3.74 3.75 3.68 3.69

0.03 0.04 0.04 0.04 0.04 0.05 0.05

0.102 0.102 0.100 0.098 0.099 0.097 0.097

0.100 0.113 0.106 0.101 0.096 0.089 0.084

1.85 2.09 1.96 1.84 1.75 1.61 1.51

0.099 0.098 0.102 0.095 0.096 0.094 0.094

4.40 4.31 4.46 4.08 4.05 3.95 3.93

0.41 0.40 0.37 0.36 0.35 0.34 0.33

5.06 4.87 4.55 4.37 4.25 4.05 3.93

Case-3

30 32 34 36 38 40 42

4.88 5.36 5.85 6.35 6.89 7.27 7.92

1.04 1.04 1.04 1.04 1.04 1.04 1.04

0.031 0.031 0.031 0.032 0.032 0.032 0.032

0.17 0.15 0.14 0.12 0.11 0.11 0.10

0.32 0.35 0.38 0.41 0.45 0.47 0.52

3.82 3.74 3.69 3.56 3.58 3.56 3.55

0.02 0.02 0.02 0.03 0.03 0.03 0.03

0.084 0.085 0.082 0.080 0.078 0.076 0.078

0.157 0.141 0.128 0.115 0.108 0.103 0.097

2.62 2.33 2.12 1.90 1.77 1.67 1.55

0.120 0.115 0.113 0.108 0.107 0.107 0.106

4.62 4.40 4.26 4.04 3.99 3.94 3.87

0.60 0.55 0.53 0.48 0.50 0.49 0.46

6.85 6.27 5.99 5.33 5.53 5.32 4.99

Cases

216

A. Koca et al. / Energy and Buildings 82 (2014) 211–221 Table 4 Heat transfer coefficient and reference temperature. Heat transfer equation

Heat transfer coefficient

Reference temperature (Tref )

qr = hr (Tref − Ts ) qc = hc (Tref − Ts ) qtot = htot (Tref − Ts )

hr hc htot

AUST Ta Top

the heated wall and the chamber depends on the temperatures of surfaces and the temperature characteristics of the indoor environment. The total heat flux between the radiant panels and the test chamber was calculated via Eq. (3) qtot =

˙ p Tw mC − qout (W/m2 K) A

(3)

The reference temperature for the calculation of the radiant heat transfer coefficient is the average unheated surface temperature (AUST) calculated by using the view factors between surfaces [51,52]

n 4 AUST =

(Fs−j Tj4 )

(4)

j=1

Fig. 6. Thermal comfort measurement equipment.

Fεs−j =

Before measurement uncertainty analysis and expected errors in the measurements were calculated and presented in Table 3. Error rates wR of each independent variable used in experiments can be determined by using the following Eq. (2): In experimental studies, it is important to determine the amount of error which affects the accuracy of the results. In the evaluation of the data obtained, errors occur in two different ways. There are the inevitable errors caused by the structure of the experimental setup and measurement instruments, and there are errors caused by the negligence of the person who perform the experiments [50].

wR = ±

∂R w1 ∂x1

2

+

∂R w2 ∂x2

2

∂R + ··· wn ∂xn

2 1/2 (2)

where R is the value which should be determined experimentally and x is the independent variable that affects R. In Table 3 uncertainty analysis of heat output, radiative, convective and total heat fluxes and heat transfer coefficients are shown for each experimental case. In calculations, measurement uncertainties of thermo physical properties of water and uncertainty of panel dimensions are neglected. Analyzing the results of the uncertainty analysis, it is observed that uncertainties of radiative heat flux and heat transfer coefficient are lower than the other parameters. Due to calculating the radiative heat transfer analytically with Mat Lab program and separating the test chamber to 16 surfaces to obtain surface temperatures sensitively, uncertainties of radiative heat transfer is lower. Because of the sensitivity of the measuring devises are very small, uncertainty values obtained are low. The convection, radiant and the total heat transfer coefficients may be calculated on the basis of the archived measurements. These parameters are presented in Section 3. 3. Determination of the heat transfer coefficients The study is aimed at calculating the heat transfer coefficients of a radiant wall heating system. The heat transfer process between

Fs−j

1 [(1 − εs )/εs ] + (1/Fs−j ) + (As /Aj )[(1 − εj )/εj ]

1 = Ai

Ai

cos i cos j

Aj

R2

dAi dAj

(5)

(6)

In the literature, instead of calculating the radiant heat transfer coefficient value from the surface of the radiant panel, it is recommended to assume it to be constant and equal to 5.5 W m−2 K−1 for heated radiant floor. [40,41] When the radiant surface temperature increases also hr may increase as indicated in the studies of the Cholewa et al. [39] and Causone et al. [46]. However the recommended radiant heat transfer coefficient in the literature do not vary so much [28,39–41,46]. The radiant heat transfer coefficient may be calculated by means of the net heat transfer among the studied surfaces and the surrounding surfaces (Table 4). The radiant heat flux among the radiant wall panels and the room surfaces was calculated by using Eq. (7) and the radiant heat coefficient through Eq. (8).

qr=

n

Fεs−j (Ts4 − Tj4 )

(7)

j=1

qr hr = = AUST − Ts

n

F (T 4 j=1 εs−j s

− Tj4 )

AUST − Ts

(8)

The walls on which the radiant heating panels were mounted were separated into 12 different surfaces as heated and unheated surfaces (surface number 5–16 as shown in Fig. 7). However in the studied room all the surfaces were separated in to 16 surfaces as seen in figure x and the total radiant heat transfer among 16 surfaces were calculated. The view factors and radiation heat transfer equations were solved by using Matlab and the results presented in Table 5. The radiation heat transfer quantities among all surfaces were calculated. Because of the fact that the current study has three different heating cases, calculations were made three times for all configurations.

217

9e−7 1.4e−7 6.2e−8 0.10 0.09 0.13 0.11 0.06 0.08 0.19

2.9e−9 1.9e−8 0.030 0.041 0.011 0.303 0.002 0.004 0.061

3e−14 0.010 0.036 0.024 0.013 0.012 0.015 0.026

0.021 0.005 0.012 0.009 0.007 0.009 0.017

3.7e−11 2.2e−8 6.6e−8 6.6e−8 8.2e−8 8.3e−8

1.7e−8 7e−8 7e−8 1.9e−7 1.9e−7

2.3e−9 2.3e−9 9.7e−8 9.7e−8

3.53e−12 1.03e−9 1.35e−8

7.52e−9 1.03e−9

3.57e−9

0.022 0.024 0.014 0.016 0.003 0.037 0.090 0.023 0.025 9.73e−8 1.90e−7 1.39e−7 3.01e−8 2.29E−9 3.57e−9 0.022 0.024 0.036 0.016 0.006 0.016 0.005 0.013 0.014 9.73e−8 1.90e−7 1.39e−7 2.29e−9 1.67e−8

15

0.008 0.009 0.005 0.007 0.000 0.010 0.200 0.005 0.006 3.52e−8 3.11e−8 1.51e−9

14 13

0.018 0.017 0.015 0.012 0.004 0.018 0.012 0.015 0.013 1.9e-8 1.2e−8 0.039 0.014 0.021 0.016 0.005 0.018 0.060 0.032 0.008 4.4e−11 0.009 0.029 0.016 0.012 1.9e−8 8.5e−9 1.9e−8 1.8e−14

0.012 0.033 0.02 0.013 0.004 0.017 0.037 0.008 0.026

9

0.047 0.016 0.028 0.021 4.8e−9 3.1e−8 4.8e−9 0.013 0.013 0.014 0.003 7.2e−12 1.2e−7 0.12 0.13 0.13 0.11 1.3e−7

6

7

8

10

11

12

0.008 0.009 0.024 0.007 0.003 0.005 0.001 0.005 0.005 3.52e−8 3.11e−8 1.51e−9 3.53e−12

16


Fig. 7. Enumerated surfaces in the studied room (volume-5).

The summation of the radiant heat transfer and the convective heat transfer values gives the total heat transfer amount as in Eq. (9). qtot = qr + qc

(9)

qc = qtot − qr

(10)

For convective heat transfer calculations, many authors formulated dependencies such as Khalifa [29–31], Awbi et al. [32–34], Karadag et al. [35–37,53] and De Carli and Tomasi [54]. The convective heat transfer coefficient between the radiant wall panels and the chamber depends on the air temperature, which was measured by a thermal comfort device at a height of 1.1 m. htot =

qtot Top − Ts

(11)

For the current study, the total heat transfer value, which depends on the convective heat transfer coefficient between the radiant wall panels and the chamber and the air temperature, was calculated by using Eq. (12). hc =

qc Ta − Ts

(12)

1.8e−8 7.2e−12 2.9e−9 1.9e−8 0.003 0.003 0.004 0.001 0.004 0.004 0.002

0.34 0.27 0.26 0.20 0.26 0.20 0.45 0.14 0.16 0.42 0.28 0.19 0.19 0.24 0.24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0.27 0.26 0.20 0.28 0.20 0.15 0.47 0.43 0.15 0.26 0.21 0.21 0.27 0.27

0.20 0.16 0.22 0.16 0.20 0.20 0.16 0.17 0.18 0.08 0.44 0.29 0.11

0.41 0.12 0.03 0.10 0.10 0.09 0.08 0.09 0.08 0.08 0.09 0.09

0.013 0.013 0.014 0.051 0.20 0.20 0.34

0.13 0.13 0.13

3 2 1 Surfaces

Table 5 View factors.

4

5

4. The findings and discussion 4.1. The heating output The measurements which were taken for each case study were used to carry out the heat transfer coefficient and are presented in Table 6 for different heated wall panel arrangements. Fig. 8 shows the radiant wall heating panel performance as a function of the temperature difference (To − Ts ) under winter environmental conditions. The heating capacity of the panel increases when the supply of the inlet water temperature increases as is expected. The data was derived both from the experimental findings in this study and the values from the standard EN 15377-1. Each case study of the data follows a different trend line due to the different heating surfaces and locations. However, the experimental findings are within the acceptable range of 15% for the present study. The heat transfer coefficient calculations for the radiant wall systems as in Eq. (1), which are given in EN 1264-5 [27] and EN 15377 [28], are often used in radiant heating and cooling engineering processes. According to the European standards, the heat transfer coefficient value must be 8 W m−2 K−1 for a heated wall.

218


Table 6 Measured and calculated parameters for a heated radiant wall. Test descriptor

1

2

3

4

5

6

7

Case 1

Tw (◦ C) Ts (◦ C) Ta (◦ C) Top (◦ C) AUST (◦ C) qr (W m−2 ) hr (W m−2 K−1 ) qc (W m−2 ) hc (W m−2 K−1 ) qtot (W m−2 ) htot (W m−2 K−1 )

31.16 22.31 17.05 17.08 17.10 28.05 5.38 16.16 3.07 44.22 8.46

32.96 23.42 17.65 17.70 17.68 31.14 5.42 17.81 3.09 48.95 8.56

34.65 24.42 18.15 18.20 18.15 34.13 5.44 19.56 3.12 53.69 8.63

36.48 25.42 18.67 18.73 18.63 37.16 5.47 21.22 3.15 58.38 8.73

38.17 26.42 19.13 19.18 19.12 39.87 5.46 23.23 3.19 63.10 8.72

40.02 27.29 19.76 19.80 19.65 42.27 5.53 24.14 3.21 66.41 8.87

41.59 28.27 20.34 20.39 20.20 44.94 5.57 25.57 3.22 70.51 8.94

Case 2


31.89 23.35 19.15 19.31 18.98 23.50 5.38 9.48 2.25 32.98 8.16

33.52 24.58 20.06 20.21 19.86 31.14 5.42 10.28 2.28 35.86 8.20

35.08 25.01 20.42 20.31 19.91 34.13 5.43 10.95 2.29 38.64 8.22

36.81 26.30 21.10 21.21 20.83 37.16 5.47 12.14 2.33 42.06 8.26

38.41 27.21 21.72 21.83 21.41 39.87 5.50 12.97 2.36 44.89 8.34

40.24 28.03 22.13 22.29 21.83 42.27 5.53 14.07 2.38 48.33 8.42

41.83 29.03 22.80 22.94 22.45 44.94 5.57 14.98 2.40 51.66 8.48

Case 3


30.91 24.80 21.59 21.74 21.71 18.47 5.98 8.32 2.59 24.65 8.74

32.76 25.97 22.39 22.58 22.50 20.89 6.02 9.33 2.61 26.79 8.76

34.67 27.20 23.29 23.48 23.37 23.19 6.05 10.33 2.64 30.22 8.90

36.41 28.02 23.66 23.86 23.77 25.76 6.07 11.60 2.66 33.52 9.01

38.42 29.63 24.95 25.20 25.05 28.10 6.14 12.54 2.68 37.36 8.98

40.20 30.26 25.34 25.57 25.42 29.84 6.17 13.29 2.70 40.63 9.17

42.18 31.84 26.51 26.75 26.61 32.68 6.25 14.53 2.73 43.13 9.19

However, the proposed equation for qtotal calculation for the heated wall depending on panels’ positions and flow temperatures in this study will be as follows: qtotal = 8.74.(Top − Ts ) Case 1

(13)

qtotal = 8.33(Top − Ts ) Case 2

(14)

qtotal = 9.1(Top − Ts ) Case 3

(15)

The heat transfer capability of the wall heating system by radiation is calculated by using Eq. (7). Fig. 9 shows the radiant heat flux densities as a function of the difference between the reference temperature (AUST) and the wall heated panel surface temperature. The findings of each case study were compared with the previous experimental studies done by Causone et al. and Cholewa et al.

Total heat flux density (W/m2)

100 90

± %15

80

[45,46]. Each graphic represents a single combination of a panel arrangement. The obtained radiant heat flux density curve is nearly linear for different case studies. Fig. 8c indicates the highest radiant heat flux density value for case 3; and Fig. 9a and b shows similar radiant heat transfer trends. The heat transfer capability by radiation amounts to approximately 68% of the total of the three different case studies. The heat transfer weights of the different case studies are shown in Fig. 10. As Fig. 10 shows, the radiation heat transfer weight reached the highest point in Case 2. In this case, the heated surface is located transverse to the coolest surface, which is the window surface. This causes a greater radiant heat transfer possibility because of the view factor between the heated surface and the other surfaces in the chamber. On the other hand, the surface which received the most heat from the heated wall panels is projected as the coolest surface, which is the window. In case 2, the first effect of the window on the radiant heat transfer weight is the greatest temperature difference with the heated wall panels and the second effect of it is that it has a greater view factor than the other cases.

70 60

4.2. The heat transfer coefficients

50 40 30 20

y = 8,74x R² = 0,99

Case 1

y = 8,33x R² = 0,99

Case 2

y = 9,10x R² = 0,99

Case 3

10

EN 15377

0 0.00

2.00

4.00 6.00 (Ts-Top) (oC)

8.00

10.00

Fig. 8. Total heating capacity for a radiant heated wall panel system.

The radiant, convective and the total heat transfer coefficient values for a heated radiant wall system were calculated in this study and also graphically presented in Fig. 11. Analyzing the data for different case studies shows that the radiant heat transfer coefficients can be considered constant for these case studies. These experimental findings are conveniently justified in the literature for a heated wall. The findings in Fig. 12 show the convective heat transfer coefficients and the comparison with the findings that were taken from

Radiant heat flux density (W/m2)


60 55 50 45 40 35 30 25 20 15 10 2.00

219

(a) Case 1 y = 5.48x R² = 1.00

qr (this study) Cholewa et al. Causone et al.

4.00

6.00

8.00

10.00


(Ts-Taust) (oC) 60 55 50 45 40 35 30 25 20 15 10 2.00

(b) Case 2 y = 5.46x R² = 0.99

Fig. 10. Radiation heat transfer weight.

qr (this study) Cholewa et al. Causone et al. 4.00

6.00

8.00

10.00


(Ts-Taust) (oC) 60 55 50 45 y = 6.11x 40 R² = 0.99 35 30 25 20 15 10 2.00 4.00 6.00

(c) Case 3

qr (this study) Cholewa et al. Causone et al. 8.00

10.00

(Ts-Taust) (oC) Fig. 9. Radiant heat flux density (a) Case1, (b) Case 2, (c) Case3.

the literature for all case studies. The measured values are consistent with the analytical studies of Churchill et al. [55] for all cases. An average convective heat transfer coefficient of 3.16 W m−2 K−1 for case 1, of 2.32 W m−2 K−1 for case 2, of 2.65 W m−2 K−1 for case 3 were derived from the present work. As can be seen in the study, the configurations of the heated panels can affect the convection heat transfer coefficient by up to %25. Furthermore, the highest convection heat transfer coefficient occurs in case 1 because of the higher surface temperatures of the radiant system. The radiant, convective and the total heat transfer coefficient values for a heated radiant wall system were determined experimentally. In Table 5, there is a comparison of the total heat transfer coefficient of the radiant wall heating with different types of radiant systems. As a result, the measured values of this study vary between 8.16 and 9.28 W m−2 K−1 . The findings of the study are compatible with that of Causone et al. [45] and EN 15377-1 [28] standard’s recommendations for a heated wall. The radiant wall heating has a lower heat transfer coefficient than the underfloor heating system while having a higher heat transfer coefficient than that of the radiant ceiling systems (Table 7).

Fig. 11. Heat transfer coefficients for a heated radiant wall panel system.

220


Fig. 12. Convective heat transfer coefficients.

Table 7 Comparison between literature and measured heat transfer coefficients for a heated wall. Source Causone et al. [45] EN 15377-1 [28] Present study

Heating type

hc (W m−2 K−1 )

hr (W m−2 K−1 )

htot (W m−2 K−1 )

Ceiling Wall Floor Ceiling Wall (Case 1) Wall (Case 2) Wall (Case 3)

0.3 – – – 3.07–3.22 2.25–2.40 2.59–2.73

5.6 – – – 5.38–5.57 5.38–5.57 5.98–6.25

5.8 8 10–11.1 6 8.46–8.94 8.16–8.48 8.76–9.28

5. Conclusion In this work, an experimental facility was developed in order to characterize a radiant wall heating allocated in a climatic test chamber. The obtained findings proved that this setup was suitable to determine the system’s heating output. Using the real size test chamber ensured the most realistic results. For this reason, the results of this study can be a reference and can directly be used in the commercial applications of the radiant wall heating systems. Further experimental evidence on the heating capacity and the heat transfer coefficients of the heated radiant wall systems was provided. The approximate average values of 5.7 W m−2 K−1 and 2.7 W m−2 K−1 were found respectively for the radiant and the convective heat transfer coefficients. Both are consistent with the data previously reported in the other works. However, an average total heat transfer coefficient of 8.4 W m−2 K−1 was obtained, which is quite higher than ones typically shown in the standard of EN 15377-1 [28]. In comparison to recommended in EN 153771 (8 W m−2 K−1 ) the difference was in the range 5–15%. So it was noticed, that the values of heat transfer coefficients for heated/cooled radiant floor, which are commonly used in practice, are underestimated, in the range of 5–15%. For a heated radiant wall, a modified equation was proposed for the calculations of the total heat transfer coefficient for the operative temperature as in (Eq. (16));

For all cases, the radiant heat transfers were dominant to the convective heat transfer as it was expected. However, according to the findings, the average radiant heat transfer ratio was 68%. Different configurations of the radiant wall systems did not affect the radiant heat transfer coefficient considerably according to the resulting radiant heat transfer coefficients which vary within a 10% range for all cases. However, a different arrangement has a much more effect on the convective heat transfer coefficient within a range of nearly 25%. The radiant wall heating systems can be good alternatives to the radiant underfloor heating systems although they have nearly 20% lower total heat transfer coefficients than the radiant floors. As different configurations of the radiant systems have more effect on thermal comfort, further studies can be conducted in accordance with thermal comfort parameters. In addition, this very research can be extended with CFD studies. Acknowledgment This study was supported by the project number TEYDEB 3100577 and financed by The Scientific and Technological Research Council of Turkey (Tubitak). This work has been a part of the project of Mir Arastirma and Gelistirme AS company department of thermo-fluid and energy research. References

qtotal = 8.7(Top − Ts )

(16)

The measured radiant heat flux densities of each case study are close to the previous experimental studies conducted by Causone et al. [45] and Cholewa et al. [46].

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