Experimental Analysis Of Fishbone Heat Exchangers In Thermoelectric Generator For Automotive Application

Transcription

1 Reviewed Paper Volume 2 Issue 12 August 2015 International Journal of Informative & Futuristic Research ISSN (Online): Experimental Analysis Of Fishbone Heat Exchangers In Thermoelectric Generator For Automotive Application Paper ID IJIFR/ V2/ E12/ 043 Page No Mechanical Subject Area Engineering Key Words Automotive Thermoelectric Generator, Fishbone, Heat Exchanger, Internal Structure Received On Accepted On Published On Kiran R. Sonawane 1 Nilesh C. Ghuge 2 M E student, Department of Mechanical Engineering Matoshri College of Engineering, Eklahare, Nashik-Maharashtra Assistant Professor, Department of Mechanical Engineering Matoshri College of Engineering, Eklahare, Nashik-Maharashtra Abstract Thermoelectric technology has revealed the potential for automotive exhaust-based thermoelectric generator (TEG), which contributes to the improvement of the fuel economy of the vehicles. Thermal capacity and heat transfer are major factors which effect on the thermal performance of TEG. As the thermal energy of exhaust gas extracted by thermoelectric modules, a temperature gradient appears on the heat exchanger surface. In order to achieve uniform temperature distribution and higher interface temperature, the thermal characteristics of fishbone heat exchangers with various heat transfer enhancement features are studied, such as internal structure and surface area. Combining the computational fluid dynamics simulations and test on an engine setup, the thermal performance of the fishbone heat exchanger is evaluated. Simulation and experiment results show that a plate-shaped heat exchanger made of brass with fishboneshaped internal structure achieves best performance than heat exchanger without internal structure, which can practically improve overall thermal performance of the TEG. Copyright IJIFR

2 1. Introduction About 40% of the fuel energy is lost to the surrounding in terms of exhaust gas which produces energy crisis and environment pollution. The expression "Energy Crisis" has become a symbol of the human concern about the increasing demands and consumption of energy on earth. For almost two hundred years, the main energy resource has been fossil fuel. The world consumption of all energy resources is forecasted to increase from 421 quadrillion Btu in 2003 to 563 quadrillion Btu in 2015 then to 722 quadrillion Btu in 2030, as shown in, as shown in fig 1. Fossil fuels continue to supply much of the increment in marketed energy use worldwide throughout the next two and half decades. Oil remains the dominant energy source, but its share of total world energy consumption declines from 38 % in 2003 to 33 % in 2030 as illustrated in Figure 2, largely in response to higher world oil prices, which will dampen oil demand in the mid-term. Worldwide oil consumption is expected to rise from 80 million barrels per day in 2003 to 98 million barrels per day in 2015 and then to 118 million barrels per day in In parallel with the improvement of the efficiency of the internal combustion engines, many researchers actively investigate the use of thermoelectric (TE) technology to recover the waste heat energy for gasoline vehicles, and hybrid vehicles. If some amount of the large waste heat could be recovered and converted into electricity, large amount of heat would be saved and the efficiency of vehicle system would increase drastically. Various thermodynamic cycles have been proposed and studied for waste heat recovery system. In absorption cooling cycles which is used in hybrid and electric vehicles transfer waste heat from the exhaust gases into the boiler of ejector for cabin cooling. The present work reviews the existing exhaust heat exchangers and optimized shape, internal structures of exhaust heat exchanger to find out best heat exchanger to get uniform temperature distribution. Figure 1: World Market Energy Consumption , (IEO, 2006) 2. Selection Criteria Of Heat Exchanger The geometry of thermoelectric module is of square plate with dimension, l*b*t Where, l: Length of TEM, b: Breadth of TEM, t: Thickness of TEM. l=b Hence, for mounting of TEMs on heat exchanger flat surface is require. Concerning this requirement we have selected two type of heat exchanger as given below: Plate shape heat exchanger and 4644

3 Hexagonal prism shape heat exchanger. Another advantages of these types of HE is that they i) Approach low temperature due to low thickness and ii) Compact in structure. 2.1 Factors to Select One Out Of These Two Type of Heat Exchanger I. Space availability: For installation of ATEG system after exhaust system less space is available. As we know that Hexagonal prism shape heat exchanger required comparatively large space than Plate shape heat exchanger. II. Height: Distance between chassis and ground is short and height of plate shaped heat exchanger is also short compare to Hexagonal prism shaped heat exchanger. The overall space requirement and height of ATEG system with plate shaped heat exchanger is less than that of ATEG with hexagonal prism shaped heat exchanger. Hence, we have selected Plate shaped heat exchanger for ATEG system Heat Exchanger s Internal Structure Optimization As a major factor, thermal capacity and heat transfer of the heat exchanger affect the performance of TEG effectively. With the thermal energy of exhaust gas harvested by thermoelectric modules, a temperature gradient appears on the heat exchanger surface, so as the inferior flow distribution of the heat exchanger. In order to achieve uniform temperature distribution and higher interface temperature, the thermal characteristics of the heat exchanger with heat transfer enhancement, internal structures are studied. In order to attempt the above concerned objective, fishbone and Scattered heat exchanger gives uniform temperature difference and high interface temperature with low back pressure. Hence, we have selected this two internal structure for our plate shape heat exchanger. 3. Design of Baffles for Fishbone Heat Exchanger By applying fundamental formula of heat transfer Ф=hAΔT, heat convection can be greatly strengthened by the increase of the heat transfer area A. This target can be approached by changing the structure of the conduction surface by fitting baffles i.e. fins. Another approach is to increase the heat transfer coefficient h. According to the fluid dynamics theories, under the condition of Reynolds number Re > 4000, turbulent fluid flow is a significant impact factor on improving the heat transfer. Moreover, the greater the heat transfer coefficient h, the better the heat transfer quantity. The thermal resistance of turbulent flow convective mostly exist in the boundary layer. The field synergy principle was proposed as another indication of the synergy degree between velocity and temperature field for the entire flow and heat transfer domain, the better the synergy was between the temperature and velocity field, the better the heat transfer. According to both theories above, the strengthening of the heat transfer can be approached by adding turbulence device to enhance the fluid disturbance and damage the boundary. Hence, dimensions of fins for fishbone and scattered is set on trial and error bases keeping in mind to have low back pressure and simplicity in manufacturing with low material cost and having better optimum turbulence, with arrangement of fins in symmetry. 3.1 Arrangement of Baffeles for Fishbone Type Plate Heat Exchanger Number of fins = 14mm, 4645

4 Length of fins = 25mm, Thickness of fins = 2mm, Height of fins = 11mm. Orientation of fins are kept concerning increase in time period of exhaust gases in heat exchanger to utilize more heat and add additional heat by turbulence, to increase the interface temperature as shown in figure. Figure 2: Design And Orientation of Fins In Fishbone Type Plate Heat Exchanger. 4. Experimentation and Result Analysis 4.1 Experimental Setup We assembled the heat exchangers with the sandwich arrangement of TEG modules between them as shown in fig. Before assembly we applied the thermal grease on both the surfaces of TEG modules to enhance the heat transfer. Insulation of glass wool with POP for binding is provided on hot side except on mounting surface of modules as shown in figure. We made use of four scales of iron with holes drill at ends for fitting of nut bolts for clamping of heat exchangers. Thermocouples (K-Type) are connected along with the display for measurement.. Figure 3: Experimental Setup 4646

5 Table 1: Engine Specification Items Engine (Gasoline) Engine type Four stroke, Three cylinder Bore (mm) 68.5 Stroke (mm) 72 Compression ratio 8.8/1 Fuel system Petrol (MPFI) Cooling Water cooled Engine working temperature ( C) 120 Sr.No 4.2 Result of Fishbone Type HE Table 2: Experimental Result Table of Fishbone Type HE. Engine Speed (RPM) Temp. Drop Voltage Current P in P out AETEG Overall Efficiency ( T ex - T in ) = ΔT V I m ex *Cp*ΔT η= P out /P in ( o C) (V) (A) (W) (W) (%) Simulation of the Thermal Field of the Heat Exchanger 5.1 Simulation model The plate-shaped heat exchanger of TEG is connected to the exhaust pipe of diameter 36 mm on both sides. The section of the plate-shaped exchanger of 5 mm thickness is a 120-mm-long by 60- mm-wide rectangle. There are 3 TMs placing on front and back surface of heat exchanger. The noslip boundary conditions are used at all the solid walls. As illustrated in Table 11 the inlet boundary condition is a uniform flow of velocity 15.2 m/s and temperature is 300 C. The exit of the exchanger is connected to the end of the rear muffler and this exit of heat exchanger is connected to atmosphere. Additionally, brass has good thermal conductivity and heat convection. The heat exchangers are made of brass, so the coefficient of convective heat transfer between the outer surface of the exchanger and the air is set to 15 W/ (m2 K). For our convenience, we used a fixed value of convection heat transfer coefficient h = 15 W/ (m2 K) in all the simulations. Figure 4 Simulation of the heat exchanger with Fishbone shape. 4647

6 Parameter Table 3: Boundary Parameter Value Engine Revolution 3500 r/min Exhaust inlet temperature 300 C Exhaust flow speed 15.2 m/s Each modules area 0.030*0.030 (m 2 ) Ambient temperature 25 C Ambient heat transfer coefficient 15 (W/(m2 K)) Material of heat exchanger Brass Dimension of heat exchanger 0.12*0.06(m 2 ) 6. Conclusion An Automobile Exhaust Thermoelectric System was designed and developed for the waste heat recovery of an automobile engine. It is found that Automotive Thermo Electric Generator is an best option for waste heat recovery from I.C.Engine. At high vehicle speeds, the total power that could be extracted was increased. More power could also be extracted by improving the exhaust gas heat exchanger. However with the current design the hot junction temperatures at or above 250 o C were allowed for the given material of TEG (Bi-Te) and results were obtained. Fishbone type plate heat exchanger is more desirable. It gives high interface temperature and uniform temperature distribution. Fishbone type heat exchanger has low back pressure. It is most effective to use Fishbone type plate heat exchanger for ATEG system which give high power output of module than heat exchanger without internal structure. References [1] Khalid Mohammad Mohiee El Dein Mansour Saqr Design and Simulation of An Exhaust Based Thermoelectric Generator (Teg) for Waste Heat Recovery in Passenger Vehicles pp 2-6, (2008) [2] Andrew P. Freedman, A Thermoelectric Generation Subsystem Model for Heat Recovery Simulations, pp 13-16, 24,31, ,M.S. Thesis, Rochester Institute of Technology (2011) [3] Chaung Yu, K.T. Chau, Thermoelectric Automotive Waste Heat Energy Recovery Using Maximum Power Point Tracking, Journal of Energy Conversion And Management (2009) [4] Jorge Vazquez, Miguel A. Sanz-bobi, Rafael Palacios, Anteneo Arenas, State of the Art of Thermoelectric Generators Based on Heat Recovered From The Exhaust Gases of Automobiles, Universidad Pontificia Comillas, Spain (2008) [5] Francis Stabler Automotive Thermoelectric Generator Design Issues, DOE Thermoelectric Applications Workshop. [6] C. Ramesh Kumar, Ankit Sonthalia, Rahul Goel, Experimental Study on Waste Heat Recovery from An Internal Combustion Engine Using Thermoelectric Technology Center of Excellence for Automotive Research, VIT University, Vellore, India (2011) [7] K. M. Saqr1, M. K. Mansour and M. N.Musa, Thermal Design of Automobile Exhaust Based Thermoelectric Generators: Objectives And Challenges, International Journal Of Automotive Technology (2007) [8] V Ganesan, Internal Combustion Engines, pp 576, Third Edition, pub.-tata McGraw-hill (2009) [9] R K Rajput, Heat and Mass Transfer, Third Edition, pub.-tata McGraw-hill (2009) Sonawane K.R & Prof. Ghuge N.C, Hydrogen (H2) Fuelled I.C. Engine-An Overview International Journal Of Informative & Futuristic Research, Vol.2(3),Paper ID:IJIFR/ V2/ E3/ 023 Page No