heat transfer augmentation on ic engine waste heat

International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 1, January 2018, pp. 924–933 Article ID: IJMET_09_01_101 Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=1 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication

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HEAT TRANSFER AUGMENTATION ON IC ENGINE WASTE HEAT RECOVERY SYSTEM Inakollu Bhanu Harsha, Kasyap Addepalli and K Karunamurthy* School of Mechanical and Building Sciences VIT Chennai Campus, Chennai, India ABSTRACT The waste heat available from the engine exhaust has potential to recover and utilise the thermal energy available. In the era of global warming it is mandatory to concentrate on the waste heat recovery. In this research article a waste heat recovery device was fabricated, to provide process heat in the form of hot water. An attempt was made to augment the rate of heat transfer between the exhaust gases and the heat transfer fluid by providing tube inserts in particular twisted tapes. The heat exchanger tube and the twisted tapes are made of copper, with multipass design. Twisted tapes are well known passive heat transfer enhancement technique. Keywords: Waste heat recovery, Heat transfer enhancement, IC engine exhaust, twisted tape inserts. Cite this Article: Inakollu Bhanu Harsha, Kasyap Addepalli and K Karunamurthy, Heat Transfer Augmentation on Ic Engine Waste Heat Recovery System, International Journal of Mechanical Engineering and Technology 9(1), 2018. pp. 924–933. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=1

1. INTRODUCTION There are many techniques for augmentation of heat transfer rates inside a circular/non circular flow passages; a wide variety of inserts has been utilized, particularly when turbulent flow is considered. The inserts studied included coil wire inserts, brush inserts, mesh inserts, strip inserts, twisted tape inserts etc. Augmentation of heat transfer in internal flows with tape inserts in tubes is a well-recognised technique employed in various industries. Waste heat recovery is the best ways of using the energy in to useful work in order to reduce the rate of fossil fuel consumption. This also helps in controlling the pollution. Internal combustion engines consume major part of the fossil fuels. The waste heat rejected as exhaust gases result in degrading the environment and rise in entropy, so it is required to utilize waste heat into useful work. The recovery and utilization of waste heat reduces the consumption of fossil fuels and emission of greenhouse gases. The active source of automotive energy for the past century is obtained from Internal Combustion Engines. This increased the need for complex and sophisticated designs concern to high fuel costs and foreign oil necessities. Many techniques such as enhanced fuel-air mixing, turbo-charging, and variable valve timing

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Heat Transfer Augmentation on Ic Engine Waste Heat Recovery System

in order are implemented to increase thermal efficiency. The exhaust energy sums up to 6070% of the total fuel energy which also increases the emissions. These raised a reason for engine manufacturers to limit various specifications of combustion such as temperatures and pressures lowering efficiency gains [1]. Most common used power source for machinery critical to the transportation, construction and agricultural sectors, engine sums more than 60% of fossil oil. The exhaust emission levels are focused on the amount of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) left after the combustion. The best ways to deal with these problems is the conservation of energy on engine since it can increase the utilization efficiency of engine and reduces emissions [2].Thus the importance of increasing fuel consumption and emissions of engine became a striving problem for which efficiency methods are required for reducing both. Among various attempts of Scientists and engineers to successfully improve engine thermal efficiency, including supercharge, lean mixture combustion, etc. Engine exhaust heat recovery is considered to be one of the most effective. Waste Heat Recovery from engine exhaust has the potential to decrease fuel consumption without increasing emissions, and recent technological advancements have made these systems viable and cost effective [3]. This paper gives a comprehensive review of the waste heat from internal combustion engine, waste heat recovery system and methods of waste heat recovery system. The Improvement in the heat exchange, by the usual or standard practice, will improve the thermal efficiency in such applications. The engineering cognizance of the need to increase the thermal performance of heat exchangers, thereby effecting energy, material, and cost savings as well as a consequential mitigation of environmental degradation had led to the development and use of many heat transfer enhancement techniques. There is an enormous database of technical literature on the subject, now estimated at over 8000 technical papers and reports, which has been disseminated periodically in numerous bibliographic reports [4], and monographs and edited texts [5]. Banglin deng et al [6] proposed a steam assisted Brayton air cycle operating on exhaust gases from Ic engine. This heat recovery system consists of regenerated Brayton air cycle and open Rankine cycle, the results indicated that the proposed waste heat recovery system is an effective method for engine exhaust. Wei he et al [7] optimized the performance of thermos electric generator using finite element method and calculated the convective heat transfer co-efficient and back pressure in engine exhaust waste heat recovery. Murat Emre Demir et al [8] assessed the performance the thermoelectric generator applied to engine exhaust waste heat recovery. The waste heat recovery system consists of a shell and tube heat exchanger whose pipes are covered with perovskite type thermoelectric materials. They also calculated the overall energy and exergy efficiency of the heat recovery. Tianyou Wang et al [9] carried out a detailed review on thermal exhaust heat recovery with Rankine cycle. Marco Soffiato et al [10] investigated to recover the waste hear available from 3 engines among 4 electrically driven Liquefied Natural Gas carrier results indicated that maximum power output achieved by 2 staged organic Rankine cycle is double that of simple Rankine cycle. It is also inferred the complex structure will have reliability issues and also economic feasibility. In a flat tube heat exchanger passive tube inserts were provided to enhance the heat transfer by Pourya Foroghi et al [11]. An industrial designed tube geometry with internal bumps were considered for analysis and investigated numerically by solving Navier-stokes and energy equation. The boundary layer modification is the main heat transfer enhancement mechanism and better results are obtained for laminar and turbulent regimes. Aluminum wire meshes of 10PPI, 14PPI, 20PPI and three different diameters of aluminum wires were used for fabrication of a heat exchanger. Experiments were carried out on heat transfer characteristics on plain tube heat exchanger, sprayed tube wire heat exchanger and tested. Results proved sprayed wire heat exchanger enhanced the rate of http://www.iaeme.com/IJMET/index.asp

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heat transfer compared to plain tube exchanger according to Yanchen Fu Jie Wen et al [12]. Enhancing the heat transfer in latent heat thermal energy storage system using extended surfaces and thermal conductivity enhancement was critically reviewed by Nasiru I. Ibrahim et al [13]. The multi objective optimization along with artificial neural network was supplied to determine the thermo-hydraulic performance of a heat exchanger fitted with vortex rods. It is found that the vortex rod with diameter ratio of 0.058 and inclination angle of 57.057 at Re = 426.767 gave best thermos-hydraulic performance as per the study carried out by Nianben Zhen et al [14].

2. EXPERIMENTAL SETUP The heat exchanger coil is made of copper tube of 0.5” NB (10.2 mm ID and 12.7 mm OD) and attached to the IC engine exhaust pipeline. A casing to cover the heat exchanger coil is fabricated using GI sheet and connected to the exhaust pipe line of the engine with help of an expander. The detailed specification of the diesel engine used for the experimentation is provided in Table 1. Table 1 Engine Specification Engine Make Fuel Cycle Cylinders Bore Stroke Speed Max. Power Compression Ratio Cooling Max Load Load Applied Type of Loading

Kirloskar AV1 Diesel 4 Stroke 1 80.5mm 110mm 1500 rpm 5 hp @1500 rpm 16.5:1 Water Cooled 25 kgf 20 kgf Mechanical loading

Water is made to pass through the heat exchanger coil to exchange heat with the exhaust gases flowing outside the heat exchanger coil. The flow rate of water flowing through the heat exchanger coil is maintained constant as 0.5 kg/min and its corresponding Re is 23,288. The temperatures of the exhaust gases and water at the inlet and outlet of the heat exchanger are measured with the help of ‘T’ (Copper-constantan) type thermo couples and digital temperature indicator. During the experimentation the engine is subjected to a constant load of 20 kgf. The total length of the heat exchanger coil is 1m and the length of the heat exchanger casing is of 1.5m.


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Figure 1 Waste heat recovery system connected to the exhaust.

3. HEAT TRANSFER PROCESS The mode of heat transfer in this heat exchange process is predominantly by convection and it depends on the heat transfer coefficient ‘h’. In order to augment the rate of heat transfer in the waste heat recovery system, passive heat transfer enhancement technique such as twisted tapes were placed in the flow passage of the heat transfer fluid. These twisted tapes creates swirl in the flow there by a secondary flow is created along with the primary flow. This swirl in the flow enhanced the Nusselt number and there by increases the rate of heat transfer.

3.1. Heat Exchanger Coil (Copper Tubes) Two numbers of copper tubes of heat exchanger coil are fabricated in order to perform the experimental analysis of heat transfer i.e with and without twisted tapes.

Figure 2 3-D Model of Heat Exchanger Coil

The copper tube was filled with sand and bent in the form of ‘U’ and finally brazed to form the required multi pass profile. The first heat exchanger setup is without twisted tape inserts whereas the second setup is provided with twisted tapes. The twisted tapes were made of copper sheets of 1mm with a twist ratio (Y) of 7. The profile of the heat exchanger coil used in the experimentation is illustrated in Figure 2.

3.2. Twisted Tape Inserts Providing twisted tapes in the flow passage are well-known heat transfer enhancement technique. These inserts enhances the heat transfer by sacrificing the pumping power of the heat transfer fluid as the pressure drop along the flow is more due to the presence of inserts. http://www.iaeme.com/IJMET/index.asp

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As such a variety of heat transfer and pressure drop correlations are available in the public domain. Common awareness of twisted tapes is that the enhancement is achieved by inducing swirl flow of the tube side fluid, resulting in higher near wall velocities and mixing of fluids thereby enhancing the heat transfer coefficient. A reasonable flow velocity is required in order to induce effective swirl flow, for that reason twisted tapes are most effective in turbulent flows with limited pressure drop. Under laminar flow conditions the improvements achieved are limited.

Figure 3 Twisted Tape Inserts

3.3. Twisted Tape Design The twisted tapes are designed according desired twist ratio in order to vary the flow rate of water. For the better twists we took the copper sheet and made it into several 2.5inch strips and twisted them taking the twist ratio 7 with the help of pliers. The dimensions of the parts are as follows: Twisted Tape dimensions: Length – 63.5mm Width - 6mm Twist Ratio – 7 Others: Plastic pipe (ID = 12mm), bolts.

4. METHODOLOGY AND EXPERIMENTATION The length of the pipes is decided from the calculations and they are bended in the form of mesh so as to increase the heat transfer rate. The pipes are bended without wrinkles by stuffing sand into the pipes so that the bending goes smoothly. Pipes carrying the water are inserted with twisted pipes to increase the heat conduction. The output temperature of the exhaust gas varies depending on the load as we are fabricating the heat exchanger for the CI engine in thermal engineering lab. So in order to measure the variable temperature, thermocouples are used. Table 2 Bill of Materials Sl No

Part Name

1

Copper pipe

2

Sheet Metal housing


Description Inner diameter : 10.2 mm Outer diameter: 12.7 mm Inner diameter: 28 mm Outer diameter: 30 mm

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3

Cooling water pipe line

4

Copper Sheets for twisted tapes

300 mm length Inner diameter: 10 mm 100 mm (L) x 6 mm (W) x 1 mm (thick), twist ratio:7

1 16

The twisted tapes are fabricated by taking thin copper sheet and twisting it to form swirl so that it makes the flow of water turbulent and the area of contact increases. The material of the twisted tapes can be modified in order to rate, we have taken copper twisted pipes due to its properties. The twisted tapes are then inserted throughout the mesh of pipes and the twist can be modified i.e. either clockwise or anti-clockwise for effective heat transfer. The entire setup is enclosed in a housing capable of holding the mesh in between so that the exhaust gas flowing through it effectively heats the water. The water is passed in a direction perpendicular to the flow of gas and two openings are provided to housing in order for the pipes to enter and leave housing. The housing can be made of sheet metal as it should also be cost effective. This entire setup can be attached to the exhaust pipeline by using flange or fabricating it to exactly mate with the diameter of the pipeline. The engine was operated at its max load condition and the exhaust gas temperature was higher. Hence the water outlet temperature will also be more as the heat transfer will be more. Water inlet is given to the copper pipes through a plastic pipe sealed with m-seal with the copper pipes. When the exhaust gas enters through the housing in which the entire copper tube mesh is placed, it heats the water flowing inside the copper tubes and heated water comes out through the outlet at the end of the housing. The heated water can be used for other purposes.

5. DATA REDUCTION The Exhaust Gas temperature (Tgi): 300° C For temperature difference of 50°C Inlet water temperature (Twi): 30°C Outlet water temperature (Two): 50°C By energy balance equation, Mg CPg(Tgi-Tgo) =Mw CPW(Two-Twi) Mg = Mf+ Ma Where, Mg- Mass flow rate of gas (kg/s) Mf- Mass flow rate of fuel (kg/s) Ma- Mass flow rate of air (kg/s) Values of specific heats considered are: Cpw = 4178 J/kgK, for water Cpg= 1115.75 J/KgK, for gas and air The Overall heat transfer coefficient (U) is given by

(1)

(2) For internal (Turbulent) flow Nusselt number (Nu) is given by, Nui = 0.023 Re0.8Pr0.4


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Where, Pr- Prandtl number Re- Reynolds number Re = Standard copper pipe dimensions selected (3/8) inches Normal Bore, ID=10.2mm OD=12.7mm To check turbulence 10-3 = 23287.67

Re= Pr=4340 Nui=128.95= hi =7939.27

For flow over cylinder (external flow) Nu0 = c Rem Pr1/3 C=0.193 M=0.618 Nu0 =270.655 Thus, h0=980.32 W/m2K U=827.57 w/m2K To calculate length of the copper pipe, Q=Mg CPg(Tgi-Tgo) =416.2863 W, We also know that, Q=U.As.LMTD To find LMTD, LMTD=

(4)

,

(Tgi-Two)=250 (Tgo-Twi)=220 LMTD=234.680 The correction factor for multi-pass flow is 0.1, Thus LMTD = 23.468 From equation 4 As= , So l=0.59m So the effective length of pipe is 0.59m. Heat lost by exhaust gas Q =Mg CPg(Tgi-Tgo) Heat gained by water (Q), Q =Mw CPw (Two-Twi) http://www.iaeme.com/IJMET/index.asp

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Maximun rate of heat transfer (Qmax) Qmax = Cmin. max Effectiveness( ) of heat exchanger is, =

(5)

6. RESULTS AND DISCUSSION In case of the twisted tape inserts, the mean Nusselt number is increased when compared to that in case of plain tube. The experimental results clearly show that the twisted tape inserts impart swirl flow and cause pressure gradient in radial direction. The increase in radial swirl and pressure causes the boundary layer along the tube wall to be thinner which results in more heat flow through the fluid. Swirl causes turbulent flow, which led to improved convective heat transfer. The effect caused by twisted tape inserts decreases at low Reynolds numbers due to weakening of swirl and less flow velocities. Thus, the increment in Nusselt number was low at smaller Reynolds number and more at higher Reynolds number. This phenomenon relates to the speed of the swirl-flow. Temperature at Engine Exhaust(oC)

350 300 250 200 150 100 50 0 0

10

20

30

Dynamometer load(kgf)

Figure 4 Exhaust temperature Vs load

The advantage of heat exchanger is that the waste heat can be recovered easily and effectively and can be redirected to Dish washers and washrooms according to hotel plan. The comparison of heat transfer between normal heat exchanger to that of heat exchanger with twisted tape inserts is depicted in the graphs. The temperature variation trends of both exhaust gas and water at inlet and outlet to the heat exchanger is also plotted in the figures 4, 5 and 6. The exhaust gas temperature at the outlet of engine exhaust for varying load on the dynamometer of Kirloskar AV-1 engine set is shown in Figure 3. The variation of temperatures of water at inlet and outlet are depicted in Figure 4, and the trends of variation of temperatures of exhaust gas at inlet and outlet of heat exchanger is illustrated in Figure 5. The two cases which are considered in the graph are Case1: When normal heat exchanger is used without the twisted tape inserts. Case2: When heat exchanger with twisted tape inserts is set up.


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Figure 4 Exhaust temperature Vs Time

Figure 5 Outlet water Temperature Vs Time

7. CONCLUSION Experimental study of enhancement of heat transfer in a heat exchanger with twisted tape inserts has been carried out to recover the waste heat from an IC engine. It can be observed that the swirl flow helps to decrease the boundary layer thickness of the flow and increase residence time of hot water in the inner tube. Higher pressure drop was observed due to the geometrical division and blockage of the tube by twisted tapes. Generation of secondary fluid motion by the twist of the tape and the resulting effect of twist improves the convection heat transfer thus improving the efficiency of heat exchanger.

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