| Summary: | 博士 === 國立成功大學 === 機械工程學系碩博士班 === 101 === This paper investigates the three-dimensional thermoelectric generator (TEG) are attached to a rectangular chimney used for venting flue gas from either a boiler or stove. The thermoelectric module consists of a hot plate, a flat spreader (or a built-in fin heat sink), a thermoelectric generator and a cold plate based on water cooling. In a thermoelectric analysis, the three-dimensional governing equations of heat and electric current in TEG at steady state are based on the conservation of energy and current. In addition, the flue gas flow is assumed to be a three-dimensional, steady, turbulent flow which is solved by mass, momentum (Reynolds-averaged Navier-Stokes equation), energy, turbulent - equations in the fluid region. Because of the high temperature flue gas flowing into the chimney tunnel, the radiation effect is considered. Moreover, the Simplified Conjugate-Gradient Method (SCGM) was used to search the optimal design. The approach is developed by using the commercial CFD code as the direct problem solver, which is able to provide the numerical solutions.
In terms of TEG modules with flat spreader, the optimization of TEG module spacing and its spreader thickness as used in a waste heat recovery system is investigated and solved numerically using the finite difference method along with a simplified conjugate-gradient method. The power density for a thermoelectric module is the objective function to be maximized. A search for the optimum module spacing (S) and spreader thickness (Hsp), ranging from 40mm 〈 S 〈 300mm and 1mm 〈 Hsp 〈 30mm, respectively, is performed. The effects of different operating conditions, including the temperature difference between the waste gas and the cooling water (ΔT = 200 – 800 K), and effective waste gas heat transfer coefficients (heff = 20 – 80 W/m2•K) are discussed in detail. It was demonstrated that the proper size of a heat spreader can decrease the thermal resistance and that the maximum power Pmax with a spreader can be significantly increased (up to 50%) as compared to TEG without spreader. The predicted numerical data for the power versus current (P–I) curve are in good agreement (within 8%) with the experimental data.
In terms of TEG modules with built-in fin heat sink, the study investigates the power output performance of the TEG module, three-dimensional numerical simulations combining convection and radiation effects, including the chimney tunnel, TEG modules, plate-fin heat sinks and cold plates, based on water cooling are developed and solved simultaneously. The effects of operational parameters such as the flue gas velocity (Vin = 3, 5 and 10 m/s) and flue gas temperatures (Tgas = 500, 600 and 700 K) on the flow and heat transfer are determined. The influences of the plate-fin height (Hfin) and number of fins (N), ranging from 0mm 〈 Hfin 〈 100mm and 4 〈 N 〈 8, on the power output and pressure drop are also described in detail. It is worthy of note that the net electric power (Pnet) of the TEG module was obtained using the ideal electric power (PTEG) minus the extra pumping power (Pfan). The numerical results for the power versus current (P–I) curve are in good agreement with the experimental data within an error of 9%.
In terms of louver fin, this study suggests a method for finding the optimal louver angle of a fin heat exchanger by use of a simplified conjugate-gradient method (SCGM) and a three-dimensional computational fluid dynamics model. The search for optimum louver angles ranging from θ = 15 to 45 for suitable louver pitches and fluid input velocity are carried out for Reynolds number ReH (based on the fin spacing 1.5 mm and the frontal velocity 1 - 5 m/s) ranging from 100 to 500. The maximum area reduction of using louver surface relative to the plain surface is the objective function to be maximized. The model calculates optimum performance of the heat-exchanger by means of finding the fin angle which would give the biggest reduction in area of the louvered surfaces relative to plain fin surfaces required to give equivalent performance. The numerical optimizer adjusts the angle of the louvered fin toward the maximization of the performance of the heat exchanger. Additionally, the correlations of the optimal louver angle as function of Reynolds number ReH are obtained.
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