Heat Transfer Engineering
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Experimental Result on Heat Transfer during Quenching of Hot Steel Plate by Spray Impingement Santosh K. Nayak, Purna C. Mishra, Manoj Ukamanal & Rajeswari Chaini To cite this article: Santosh K. Nayak, Purna C. Mishra, Manoj Ukamanal & Rajeswari Chaini (2017): Experimental Result on Heat Transfer during Quenching of Hot Steel Plate by Spray Impingement, Heat Transfer Engineering, DOI: 10.1080/01457632.2017.1341193 To link to this article: http://dx.doi.org/10.1080/01457632.2017.1341193
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Date: 13 July 2017, At: 23:10
HEAT TRANSFER ENGINEERING , VOL. , NO. , – https://doi.org/./..
Experimental Result on Heat Transfer during Quenching of Hot Steel Plate by Spray Impingement Santosh K. Nayak, Purna C. Mishra, Manoj Ukamanal, and Rajeswari Chaini School of Mechanical Engineering, KIIT University, Bhubaneswar, Odisha, India
ABSTRACT
The present article discusses the experimental results on cooling characteristics of a stationary hot steel plate by spray impingement. The experimental setup consisted of an electrically heated flat stationary steel plate of dimension 120 mm × 120 mm × 4 mm, spray setup, water supply, and air supply unit. The effects of various controlling parameters such as air-water pressures, water flow rate, nozzle tip to target distance and impingement density were determined and analyzed. The cooling rates were computed from the time-dependent temperature history and used to analyze the parametric effects. The results obtained in the study confirmed the higher efficiency of the spray cooling system and the cooling strategy was found advantageous over the conventional cooling methods available in the open literature.
Introduction Spray Impingement cooling is one of the latest temperature control methods to achieve large heat removal rates in processes such as metal quenching, electronic chip cooling, nuclear reactor, food processing, industrial and agricultural applications. Heat fluxes as high as 10 MW/m2 with spray impingement cooling has been reported by Selvam and Ponnappan [1]. Several technologies [2–5] have been evolved recently to improve the heat transfer in metal quenching. Ultrafast cooling technology [2, 3] improved the traditional cooling rate of 30°C/s to 80°C/s, depending on the final thickness, to 300°C/s on 4mm-thick hot strip. The accelerated cooling method [4, 5] having more than 200°C/s on 3-mm thickness, makes it possible to increase the strength of steel or to achieve the same level of strength with a low carbon equivalent design. These technologies use larger flow rates than conventional cooling methods (such as jet or water column cooling), basically. In the ultrafast cooling, total water flow is well known to be 17,000 L/min m2 [3] of cooling length. This corresponds to 9200 L/min m2 assuming a 1.8-m width, [4] which is more than double the maximum flow rate for the conventional runout table (ROT) cooling [3]. Bhattacharya et al., [6] reported that ultra-fast cooling in the hot strip mill, entails cooling rate of about 300°C/s, which corresponds to heat transfer rate of 4.37 MW for a 4 mm thick carbon steel strip. The cooling rate obtained
was an order of higher magnitude than conventional laminar jet cooling. Investigations involving air and water flow rates and impact-water fluxes within ranges of practical interest in continuous casting (i.e., 5 to 10 g/s (i.e., 3.9 to 7.8 NL/s), 0.3 to 0.6 L/s, and 2 to 90 L/m2 s, respectively) have shown that the droplet dynamics persist in having an important influence on impaction heat transfer [7–9]. Hou et al. [10] carried out experiments on phase change spray cooling. An R22 pressure spray cooling system was designed and built. The system cooling performance was experimentally investigated with the nozzle inlet pressure in the range of 0.6 MPa to 1.0 MPa. Enhancement of cooling rate of an AISI 304 steel plate by air-atomized spray cooling with different types of surfactant additives has been studied in the present work. Ravikumar et al. [11] tested the atomized spray cooling on a plate maintained at the initial surface temperature of 900°C. They concluded that air-atomized cooling provides very high cooling rates, which lead to producing high strength steels on ROT. The heat transfer during spray cooling was studied experimentally by Zhang et al. [12] on one flat and three enhanced surfaces using deionized water for flow rates from 22.2 L/h to 60.8 L/h, orifice-tosurface distances from 0.5 cm to 3.0 cm and spray inclination angles from 0° to 45°. Spray cooling technique is mainly used in strip investment, plate mill and mill cooling [13] quenching of
CONTACT Dr. Purna C. Mishra
[email protected] School of Mechanical Engineering, Kaveri Campus, KIIT University, Bhubaneswar – , Odisha, India. Color versions of one or more of the figures in this paper can be found online at www.tandfonline.com/uhte. © Taylor & Francis Group, LLC
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extrusions, forgings and continuous castings [14] and cooling of rolls in metal rolling [15]. The metallurgical industry, mostly used spray cooling for quenching of iron ingots and cooling of alloy strips in continuous casting processes [16]. Marcos et al. [17] studied spray cooling at low system pressure, which can be used in low temperature thermal control in industries such as electronics, avionics, lasers and electro-optics. Spray cooling application is also found in agriculture industry, food industry and medical application. Cryogenic spray cooling is currently used during laser irradiation treatments for removal of port wine stain birthmarks. Effectively cooling the surrounding epidermis allows for high powered lasers to be used, improving the treatment results. Spray cooling of electronics is still a developing technique, but is currently being applied in the CRAY X-1 supercomputer and the space shuttles open loop flash evaporator system. Future uses of spray cooling include cooling of high-powered electronic components for lasers, hybrid vehicles and various military applications. Mist cooling is a type of heat transfer to a dispersed flow that occurs when dispersed droplets impinge on a heated surface. The heat transfer characteristics of mist cooling are similar to those of a boiling curve, however, mist cooling is more complicated than boiling heat transfer because it is affected by additional parameters such as droplet behavior and the air flow rate [18]. Cooling by a dispersed flow is classified into either “spray cooling” or “mist cooling” according to the atomization of the droplets. In spray cooling, compressed water is atomized by the pressure at a nozzle, while in mist cooling, the droplets are atomized by compressed air [19]. Lee et al. [20] obtained the mist-cooling curve for a heated 800°C cylinder and identified two distinct heat transfer regimes from the cooling curve and the surface-wetting images obtained using a charge-coupled device camera. Based on the results, they introduced a simplified model and verified it by comparing the estimated mist cooling curve to the measured data. They have yielded two mist cooling regimes from the measured cooling curve and introduced the simplified model from the qualitative analysis on each regime. The estimated curve from the model agreed with the measured one in cooling regime II and showed a similarity in cooling regime III. Fukuda et al. [21] studied the effects of droplet diameter, surface roughness, and impinging velocity on the behavior of a droplet impinging on a hot surface. They used surface samples of cylinder blocks of stainless steel having four different degrees of roughness (Ra ), i.e., Ra = 0.04, 0.2, 3, and 10 microns in the experiment. The diameter and impinging velocity were controlled independently using a micro-jet dispenser, and their values were in the ranges of 300–700 µm and 1.0–4.0 m/s, respectively.
The solid-liquid contact time was found to increase with an increase in the surface roughness and a decrease in the impinging velocity and was of the order of the selfoscillation of the water droplet. The maximum spread of the droplet decreased with increasing impinging velocity. A cooling curve was obtained for surface temperatures ranging from 500°C to 100°C, and the cooling time was found to decrease with an increase in the surface roughness of stainless steel. Moreover, the cooling effectiveness of each droplet increased with an increase in the surface roughness, a decrease in the droplet diameter, and an increase in the impinging velocity. The cooling rate increases with an increase in the impinging velocity. The effect of the velocity was confirmed and observed mainly at film boiling, which occurs when the sample temperature is greater than 300°C, and this trend was confirmed, by Choi and Yao [22]. The cooling rate increases with an increase in the impinging velocity at film boiling, whereas the impinging velocity hardly affects the cooling rate at nucleate boiling. Pereira et al. [23] investigated experimentally the nucleate boiling of sub cooled water, under 100 cm2 square arrays of impinging sprays. They used commercially available full cone pressure nozzles, of distinct flow capacities, where the average impinging coolant mass flux spanned the 0.3–7.2 kg/m2 -s range. They found that the average heat flux through the heated, upward-facing, copper impingement surface to be equal to the sum of single-phase and nucleate boiling heat flux components. They obtained the noncritical-heat-flux cooling capacity and efficiencies of up to 2000 kJ/kg and 83% respectively. Bostanci et al. [24] carried out an experiment to investigate the effect of enhanced surfaces on heat transfer performance by taking micro, macro and multi-scale structure test surfaces. They conducted the test in a closed loop system using vapor atomized spray nozzles with ammonia as the working fluid and applied heat fluxes of up to 500 W/cm2 , and flow rates of 1.6 ml/cm2 .s of liquid and 13.8 ml/cm2 .s of vapor to enhance the cooling performance data. For each surface, data indicated that the multi-scale structured surface achieved the highest heat transfer coefficient of 772,000 W/m2 .°C, corresponding to 161% enhancement over the reference surface. The ammonia spray cooling, with the utilization of enhanced surfaces, offers significant cooling performance for high heat flux thermal management applications that target to maintain low device temperatures with a compact and efficient cooling system. Tao et al. [25] experimentally investigated the heat transfer in a nonboiling spray cooling system with de-ionized water as the working fluid in an open-loop test system with two full-cone spray nozzles. They found that the nonboiling spray cooling system can remove high heat flux from a small surface while
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maintaining the surface at desirable low temperature and at an optimal nozzle-to-surface distance of 1.451 × 10−2 m. Increasing the liquid volume flow rate or reducing the liquid inlet temperature significantly increases the heat transfer coefficient and can be enhanced by adding a surfactant to the working fluid with an appropriate concentration. Kim and Lee [26] investigated experimentally the thermal and hydrodynamic behavior of a water drop impinging on heated porous surfaces having different permeability and surface roughness. The surface temperature was varied from 60°C to 300°C and the impinging velocity was varied from 0.8 m/s to 2.3 m/s keeping the drop diameter fixed at 2.6 mm. Larger impinging velocity resulted in higher transition temperature from the contact regimes to the noncontact regimes, which is due to the increase of the impact pressure at the liquid–solid interface. The spreading and wet-diameter ratios, and the time for complete permeation turned out to be the major indicators of the cooling performance, which were strongly influenced by both the impact condition and the structural characteristics of the porous substrates. Thiagarajan et al. [27] compared the spray boiling heat transfer performance of a conductive micro porous copper surface to that on a plain surface by using two full-cone spray nozzles spanning a range of volumetric flow rate from 1.1 cm3 /s to 15.8 cm3 /s and degassed HFE7100 as the coolant. They experimentally investigated that spray impingement on the micro porous surface has an enhancement of 300–600% in the heat transfer coefficient at a given wall superheat compared to spray impingement on a plain surface. The critical heat flux also increased by up to 80% in the case of spray impingement on a micro porous coated surface as compared to impingement on a plain surface, depending on the flow rates and the subcooling levels. Galvan et al. [28] investigated the effect of the spray cone angle on heat transfer and film thickness using the dielectric refrigerant R134a for different flow rates with low surface roughness. The results revealed that as the spray cone angle decreases, the thermal performance worsened because of a delay in the onset of the nucleated boiling regime. In the nucleate boiling regime, film thickness increases as the cone angle decreases. Guo et al. [29] used two spray nozzles with various combinations of water pressure and temperature for quenching of heated thin sheet of 2024 aluminum alloy. Inverse heat conduction technique was applied for calculation of surface heat flux and the results revealed that the water pressure plays vital role upon enhancement of thermal performance. Nayak et al. [30] experimentally studied the influence of variation of spray parameters on heat transfer enhancement by using an internally mix full cone air atomizing nozzle. Experiments were conducted for different specimen thickness and found maximum surface
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heat flux of 4895.525 KW/m2 for 4 mm steel plate at 4 bar water pressure, 3 bar air pressure and for 120 mm nozzle to plate distance. Cebo-Rudnicka et al. [31] carried out an experiment on spray cooling of cylindrical brass and inconel specimen and investigate the effects of several controlling parameters such as water pressure, mass flux and nozzle height for enhancement of heat transfer coefficient. Wang et al. [32] investigated the thermal performance and wetting front propagation phenomenon of a single circular jet impinging on the top or bottom surface of a heated steel plate. The results indicate that the impinging jet velocity is the key factor to enhance the cooling performance. Mishra et al. [33] experimentally investigated the effects of various controlling spray parameters upon cooling rate during spray impingement cooling of hot steel plate using distilled water as coolant in three air atomizing nozzles. The results showed that the average mass flux and cooling rates at the stagnation zone was highly affected by water pressure and nozzle to plate spacing. Agrawal et al. [34] evaluated the quenching performance of a vertical stainless steel surface by jet impingement cooling method under various rewetting parameters. The results indicated that the surface rewetting performance was enhanced by increasing jet diameter as well as jet Reynolds number. Wang et al. [35] experimentally studied the effects of mass flux and wall temperature on heat transfer coefficient during spray cooling and developed a correlation for Nusselt number with respect to wall temperature. Pereira et al. [36] experimentally calculated the effects of square arrays of impinging sprays upon average heat flux which was found to be the sum of single phase and nucleate boiling heat flux components. Nayak and Mishra [37] experimentally determined the thermal characteristics of ultra fast cooling of heated steel specimens under different controlling spray parameters. The air assisted spray cooling method found to be an effective cooling media to enhance the heat transfer rate.
Experimental setup and procedure The schematic of the experimental setup used in the present work is shown in Figure 1. The test piece consisted of an electrically heated steel sheet of dimension (120 × 120) mm2 and 4 mm thickness. A square base electrical coil-heater of 2.5 KW capacity was used to heat the plate to the desired temperature, upper side of which was cooled by the three air atomizing nozzles. The air and water delivery pressures were controlled with the help of regulator valves. To achieve the optimal atomization, various combinations of air and water pressures were selected. The flow rate was measured with the help of rotameter supplied by Eureka Industrial Equipments Pvt. Ltd. India.
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Figure . Schematic of the experimental setup.
Centrifugal pump with the spray setup was supplied by Spraying System Company, Bangalore India. For each test condition, a simple mechanical patternator was used to measure the local and average water impingement density. The local time-dependent surface temperature distribution was measured on the bottom side embedded with K-type thermocouples at desired locations. A sample of thermocouple arrangement on the plate is as shown in the Figure 2(a) and Figure 2(b) shows the position of thermocouple tip within the test plate. For each test four thermocouple data were obtained through the data acquisition system (cRIO) supplied by NI-Systems Ltd. The obtained data were processed using LabVIEW software. The steel plate had to be heated to a temperature slightly higher than the test temperature because some amount of heat would be lost to the surroundings from the heated test plate during the transfer from heater to the test bed. Then this hot steel plate was taken out of the heater and positioned on the test bed (underneath the air atomizing nozzles) manually. The initial temperature for each test condition was recorded and then the spray was turned on. The spray was pre-set at different combinations of air and water pressures. At each combination of air and water pressure, the water flow rate was recorded directly by using the flow meter attached with the spray system.
Figure . (a) Sketch of thermocouple installation location. (b) Position of thermocouple tip within the test plate.
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The nozzle to surface spacing was considered as fixed at 42 mm. In addition to the heat transfer measurement, the local impingement density at various air-water pressure combinations was measured by using the simple mechanical patternator. From the transient time-temperature data obtained in the experiments, the maximum dimensionless cooling rate was computed and recorded for further parametric analysis.
Results and discussion The experimental data obtained concerning the rate of heat transfer has been presented to summarize the behavior of spray impingement cooling processes. Figure . Nondimensional temperature difference (T∗ ) vs. time (t∗ ) at different air pressures and . bar water pressure.
Effect of the injection point and cooling curves The injection point has great influence on the cooling profile during spray impingement cooling of a flat steel plate. The nondimensional temperature difference is computed as the ratio of measured temperature difference (T) and cooling water temperature (TC ), while the nondimensional cooling time is obtained from eq. (1). T ∗ =
T l2 and t ∗ = TC ∝
(1)
The possible influence of the injection points are shown in Figures 3 and 4, where the dimensionless plate surface temperature difference (T∗ ) is plotted with respect to dimensionless cooling time (t∗ ) for different air pressure and at water pressure of 3 bar and 2.5 bar respectively. It is observed from Figure 4 that the central thermocouple shows an interesting cooling trend between dimensionless temperature difference (T∗ ) and cooling time (t∗ ) during air-water spray cooling. From the point
of injection there is a rapid drop of temperature as the cooling time passing. At different values of air pressure the temperature difference profiles show different trends from which the contribution of air in the spray cooling can easily be identified. It is clear that at combined air and water pressures at 2.0 bar and 3.0 bar, the temperature distribution is sharp while at 3.5 bar air pressure and 3.0 bar water pressure, it is relatively flattened. This is due to the fact that with increase of air pressure to a higher value the atomization also increases, but the water flow rate decreases, decreasing the heat transfer and the tiny water droplets evaporate before falling on the plate. Thus, the air-water pressure combination is to be optimized to achieve the optimal heat transfer rate, which is also evident from Figure 4. From Figure 4, the cooling effect is clearly understood for air-water mixed spray impingement cooling. In the absence of air (i.e., Pa = 0.0 bar), the convection process is only possible due to water and the dimensionless temperature difference profile greatly changes.
Cooling rate (CR) From the measured dimensionless time dependent temperature distribution curves at each thermocouple location, the corresponding maximum value of cooling rates were computed by taking the peak values of temperature and time and using eq. (2). CR =
Figure . Nondimensional temperature difference (T∗ ) vs. time (t∗ ) at different air pressures and . bar water pressure.
T1 − T2 ◦ C/Sec2 t 2 − t1
(2)
Further, the cooling rate in eq. (2) is nondimensionalized by using eq. (3). l 2 T T ∗ ∗ = (3) CR = t∗ αTC t
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The highest and lowest values of dimensionless cooling rate (CR∗ ) obtained in the experiments were 13.65 and 5.89, respectively. Effect of impingement pressure Figure 5 shows the effect of air pressure on the dimensionless cooling rates achieved for different fixed water pressures. From Figure 5, it could be observed that the highest value of nondimensional cooling rate (CR∗ ) obtained for the central thermocouple at the 1.0 bar air and 2.5 bar water pressure combination is 9.104, whereas the highest value of nondimensional cooling rate of central thermocouple at air and water pressure combination of 2.5 bar and 3.0 bar is 14.67. Experimental results reveal that the more is the air pressure; there is a significant change in the cooling rate at a fixed water pressure. Hence, mixing of air with water has a predominant role in enhancing the heat transfer rate from the steel plate surface during spray quenching. As the pressure of the cooling water increased at a constant air pressure, the flow rate of the water increased. The mass flow ratio increased with increasing water pressure or decreasing air pressure. Effect of water flow rate Figures 6 represent the effect of water flow rates to the dimensionless cooling rates at the location of central thermocouple on the steel plate surface for different water pressure values. From Figure 6, it could be observed that, there was no definite trend between cooling rate and water flow rate. This was because of the fact that in this case cooling was caused both by air flow and water spray.
Figure . Water flow rate versus nondimensional cooling rate (CR∗ ) at central thermocouple (TC) for different water pressures (Pw ).
The contribution of both air and water were significant and responsible for cooling effect. Hence water flow rate alone cannot be used to improve the cooling rate. High flow rate in a spray quench accelerates cooling but there still exists the “limit” of the cooling rate in each specific operating condition. The nature of the distribution shows that an optimal value of the dimensionless cooling rate exists. The optimal value of cooling rate can be obtained through optimization of water flow rate, i.e., optimizing the air-water pressure values. The low was the air pressure, the higher water flow rate achieved and less atomization appeared. Whereas, for the very high values of air pressure, the water flow rate values were low and atomization was high. From Figure 6, it is clear that for higher value of water pressure (Pw ), the cooling rate was enhanced. The calculated water flow rate for the corresponding air pressures is depicted in Tables 1 and 2. In one set of experiments, for a fixed water pressure of 2.5 bar and variation of water flow rate from 300 LPH to 500 LPH, the highest nondimensional cooling rate (CR∗ ) achieved at the centrally place thermocouple was ranged from 6.68 to 9.1. While, in the other set of experiments, with 3.0 water pressure and water flow rate in the range of 190 LPH to 420 LPH, the nondimensional cooling rate Table . Test matrix for water flow rate at different air pressures and . bar water pressure. Ambient Temp. = °C; Nozzle to Plate Distance = mm; Initial Spray Water Temperature = °C; Time of Spray = sec.; No. of Nozzles on = Test
Figure . Air pressure versus nondimensional cooling rate (CR∗ ) at TC (central thermocouple) for different fixed water pressures.
Air Pressure (Pa ), bar
Water Flow Rate, LPH
. . . . .
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Table . Test matrix for water flow rate at different air pressures and . bar water pressure. Ambient Temp. = °C; Nozzle to Plate Distance = mm; Initial Spray Water Temperature = °C; Time of Spray = sec.; No. of Nozzles on = Test
Air Pressure (Pa ), bar
Water Flow Rate, LPH
. . . . .
(CR∗ ) obtained was in the range of 10.4 to 13.6. Hence, it can be predicted that the optimal value of cooling rate could be achieved by optimizing the water flow rate during spray cooling through appropriate combination of air and water pressure values.
Surface- wetting phenomena The wetting front movement was captured by the help of a digital camera as shown in Figure 7. It was observed from the picture that the wetting front moved from center of the plate to the edges smoothly might indicate the instance of surface wetting corresponded to the inflection in the cooling curve measured in the experiment. Therefore, the difference between the cooling curves at the three thermocouples was caused by the different times at which the surface-wetting phenomenon occurred. Surface wetting occurs when mist droplets penetrate into a thermal
Figure . Impingement density measurement concept.
boundary layer and reach a heated surface. The faster and the more the mist droplets there are, then surface wetting temperature will be higher. Impingement density measurement Water was allowed to enter the graduated bottles through the collection tube array during spray. To ensure no leakage in collection tubes and the measurement bottles, rubber cocks are provided at the opening of each bottle. Water then passed through the tubing under the force of gravity and fell to the graduated glass bottles where the local water impingement density was measured. The volume of water in the measurement bottle was measured by visualization method as depicted in Figure 8. In Table 3, tubes 1 to 5 represent the tubes placed at different locations in the tube matrix. The local mass impingement density (MID) values were plotted against the tube positions and depicted in Figure 9. Effect of spray impingement density Figure 10 presents the data of nondimensional cooling rate with respect to mean spray density and standard deviation spray density, which show a very clear trend of increasing with mass flux. At low values of mean spray density, this nondimensional cooling rate is higher for the vertical down spray. Also, at low mean spray density values, the nondimensional cooling rate at the center location are higher than that at the side location of a Table . Local impingement densities in (lt/ m s) at air pressure = . bar, water pressure = . bar and water flow rate = LPH. Points / Tubes
Figure . Photograph of the surface wetting propagation in present research.
P P P
Position
. . .
. . .
. . .
. . .
. . .
Centre Forward Backward
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Figure . Nusselt number versus water flow rate at bar water pressure. Figure . Local impingement density versus tube distance corresponding to Table .
downward spray. It is believed that this shift in trend lines is also due to the combined spray deposition and liquid overflows. The cross flow from the side of plate causes quenching at the center location at a higher cooling rate and temperature. These observations of orientation and angle of attack (center versus side) effects do not appear in high spray density tests, since the spray impingement is the dominant factor. The cooling characteristics can be well understood by statistical analysis of cooling data. Figure 11 gives the error deviation of Nusselt number versus water flow rate at different air pressures and at 3 bar water pressure.
Uncertainty analysis Efforts were made to underrate the experimental errors for better accuracy in the investigated consequence. As per ASME test code PTC 19.8–1983 which mentions the methodology for uncertainty, an error can be measured in two components: (1) Systematic or bias error (B), due to faults in the measuring instruments involved in the investigations, and (2) Random or Precession error (S), due to the imperfection in explaining the parameters being measured, due to noise in the system [38]. When the systematic errors are very small, a high degree accuracy can be achieved in the experiments. To minimize the systematic errors, the measuring instruments are initially calibrated. In the present research work, the measured parameters are air flow rate, water flow rate and temperature. The maximum monitored errors for these parameters have been proclaimed in the Table 4. Based on veracity of rotameter and weighing machines, bias errors were found out for flow rates. The operating conditions of the centrifugal pump used include: liquid temperature ranges from 0°C to +90°C, ambient temperature up to +55°C Table . Measured uncertainty in the experiments. Relevant Parameters
Figure . Nondimensional cooling rate versus spray impingement density at fixed water pressure = . bar.
Test specimen (length, breadth and width) Nozzle height Water flow rate Air flow rate Thermocouple wire Thermocouple location Temperature measurement Cooling rate Local impingement density Nusselt number
Error Percentage B = ± .%, ± .%, ± .% B= B= B= B= B= B= S= S= S=
± ± ± ± ± ± ± ± ±
.% .% % .°C .% .°C .% .% .%
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and maximum operating pressure from 0°C to +40°C is up to 10 bars and from +41°C to +90°C is up to 6 bars. For measurement of temperatures, the thermocouples were calibrated at the boiling point of water and by pyrometer and standard thermocouple at elevated temperatures. The emblematic bias in the thermocouple reading is ±2.1°C having zero correctness error. Therefore, much attention should be taken for proper contact of the thermocouple, test plate and measured point by stuffing the thermocouple hole with an extremely conductive material to reduce the secluded air gap. Another important aspect for temperature error is data acquisition system involved in the investigation. As per proclaimed by the manufacturer, the offset in the temperature measurement is +0.7°C and sensitivity is