Thermal Performance Analysis of Compact Heat Exchangers for Thermoelectric Generators

The efficiency of internal combustion engines (ICE) is usually about thirty percent of the total energy of the fuel. The residual energy is lost in the exhaust gas, the lubrication, and the cooling water in the radiators. Recently much of the researcher’s efforts have focused on taking advantage of wasted energy of the exhaust gas. Using a thermoelectric generator (TEG) is one of the promising ways. However, TEG depends entirely on the temperature difference, which may be offered by the exhaust muffler. An experimental test has been conducted to study the thermal performance of a different muffler internal design. The researchers resort to the use of lost energy in an ICE using TEG, which is one of the ways to take advantage of energy lost, which depends on the difference in temperature. TEG needs a heat exchanger and the muffler one of its types. In this work, four different types of mufflers will be designed and studied. The results showed that the thermal performances of the studied models compared to the empty cavity were as follows, the serial plate structure 56.11%, the central Box structure 52.73%, and the central curvature structure 29.61%. The highest thermal performance is on the serial plate structure relative to the other types.


INTRODUCTION
An internal combustion engine (ICE) is one of the important prime movers driving many of the automobiles today. The exhaust gas from an internal combustion engine carries away approximately 30% to 40% of the combustion heat (J.S.Jadhao, 2013). However 60% to 65% of thermal energy of the used fuel is wasted without converting it into useful energy or work. There is a lot of heat energy goes out the engine to the atmosphere in the form of waste heat. It is important to find a method to recover this heat, which is, in turn, leads to enhance the efficiency of the engine. There are different techniques used to recover the wasted energy of the exhaust, some of these techniques are charging like turbocharging, small scale Rankin cycle, and thermoelectric generator (TEG). In general, heat exchangers are important devices used for different processes such as utilization, transferring and exchanging the thermal energy in varied applications. Any heat exchanger is considered as one of the devices to transfer heat, its transfer the thermal energy between fluids at different temperature (Basma, and Hawraa, 2019). The exhaust heat exchangers of (ICE) can be selected as the significant type of heat exchangers which can be used with TEG. To achieve this purpose, proper heat exchanger design should be selected to give high-temperature deference. (C. Q. Su, Zhan, and Shen 2012) studied the thermal characteristics of exhaust gas, designed two different internal structures and thicknesses; the two rows of fins in two sides shape and the Fishbone shape as shown in Fig.1 by changing the internal baffles order. The researcher used CFD software to simulate the exhaust gas flow. The researcher found that the Fishbone shape is better than the two rows of fins in two sides' shape.  The researcher used CFD simulation done by ANSYS V.14.to compared pressure distribution, heat exchanger, and to simulate the exhaust gases flowing inside the heat exchanger. The results showed that the rectangular-shaped with gradual increasing cross-sectional area had better uniform temperature distribution than the rectangular shaped with equally cross-sectional area. So the heat exchanger with a gradual increasing cross-sectional area of a rectangular shape is ideal for TEG.
In the present work, four different types of the muffler with different internal structures are proposed and the temperatures are measured along the surface of different types of muffler and compare them with the empty cavity one relative to the best and worst in terms of the highest rate of heat transfer and less pressure drop.

EXPERIMENTAL TEST RIG DESCRIPTION
The muffler is a part of the exhaust system that has a function of noise damping. Generally, automotive mufflers consist of an inlet and outlet pipe separated by a chamber that is cylindrical in geometry. There are four types of this chamber in commercial use, these are:  An empty cavity: The internal structure of this type is empty from the inside.  Muffler with an internal structure: This type contains internal plate fins arranged in a serial form.  Muffler with an internal structure: This type contains a central box open from the top and from the other side which is considered as the base of the box is welded.  Muffler with an internal structure: This type contains a central curvature fin with two plate fins putting from each side of the central fin. Fig.2 shows different types of mufflers.

EXPERIMENTAL PROCEDURE
After installing the test rig and the measuring devices, experimental work has been done following these steps: 1-Start the engine and waited for ten minutes until the steady-state condition was achieved. At this point the temperature along the surface of the muffler is recorded, and the pressure drop is measured at the inlet and exit of the muffler using the digital manometer and the pitot-static tube. 2-Replacing the tested muffler with the other manufactured one to repeat the same procedure of running specified in point 1 above to take the readings and collect the required data. 3-The above procedure was repeated for all the fabricated mufflers.

METHOD OF CALCULATIONS
The experimental data includes the measured temperatures and pressure drop. The air thermophysical properties were taken at 25℃. The heat transfers from the exhaust gases to the muffler and then to the ambient was calculated by using the below equation. It is used when the heat gained or lost by cold or hot fluid should be calculated and only for one type of fluid: = ̇ Cp ∆ (1) Velocity can be calculated by pitot tube law as follows: The effectiveness method offers many advantages to analyze the problems in which the comparison between different types of heat exchangers. The effectiveness of the heat exchanger can have been calculated (j_Holman) as: So the thermal performance factor for any heat exchanger system can be calculated as: = 1 2 * 100% (6)

RESULTS AND DISCUSSION
The results of experiments are presented and discussed the problem of heat transfer inside the muffler. The experimental work includes the velocity of the exhaust at the inlet of the muffler, temperature along the muffler surface, pressure, heat transfer rate, and the test section performance. Fig. 5 shows the temperature distribution for the mufflers with empty cavity, serial plate structure, central box structure and central plate structure along their outer surface for inlet temperature, = 46℃. This figure illustrates that the mufflers with the serial plate structure have a higher temperature distribution compared to the other three mufflers. The temperature difference between the inlet and the outlet of the muffler of empty cavity, serial plate structure, central box structure and central curvature structure are 5.5℃, 7.42℃, 5.8℃ and 5.7℃ respectively.     7 shows the temperature distribution for the mufflers with empty cavity, serial plate structure, central box structure and central plate structure along their surface at 50.1℃. This figure also clarifies that the serial plate structure has a higher temperature distribution compared to the other three mufflers. The temperature difference between the inlet and the outlet of the muffler of empty cavity, serial plate structure, central box structure and central curvature structure are 3.89℃, 10.33℃, 5.9℃ and 5.8℃ respectively.       Fig. 11 shows the variation of the thermal performance with the inlet temperature for the four types of mufflers. When the temperature increases, the thermal performance of the muffler increases, it is evident from this figure that the serial plate structure has the maximum thermal performance compared to the empty cavity muffler, which is 56.11%. The central box structure and central curvature structure have 52.73% and 29.61%, respectively.

CONCLUSION
The following conclusions can be given: The performance of the muffler is highly affected by the insertion of the internal structures. By comparing the collected data in the four cases (empty cavity, serial plate, central Box and central curvature structures), it is found that the insertion of those internal structures caused the highest heat transfer rate and highest pressure drop. The heat transfer improvement is finding for muffler with serial plate structures compare with the other three types. So, the serial plate structure is the best design since it has maximum heat transfer rate and second maximum pressure drop. NOMENCLATURE AETEG = automobile exhaust thermo-electric generator CFD = computational fluid dynamic ICE = internal composition engine. TEG = Thermoelectric Generator. A= Cross sectional area of air passage in test section 2 . Cp = Specific heat capacity of air at constant temperature and at atmospheric pressure J/Kg.K. D = the inlet diameter (0.05m).

REFERENCES
= total pressure, Pascal. = static pressure, Pascal. Q act. = The heat transfer rate in the studied model.