• Energy Research

Abstract Transpiration cooling is one of the most efficient cooling technologies to protect hot section components such as turbine airfoils, missile heads and shells of rockets or space craft. This external cooling method has much higher efficiency than film cooling when consuming the same amount of coolant, due to the uniformity of coolant distribution. However, pore plugging, which frequently occurs during the operation of transpiration cooled components, has limited its long term stability and prevented its application in industrial components. Dust deposition is one of the main reasons causing plugging of pores for transpiration cooling. Although a lot of effort has been devoted into explaining dust deposition and erosion mechanisms of transpiration cooled components, reducing plugging impact remained difficult as the plugging caused by dusts was unpredictable for traditional porous media. Additive manufacturing, with capability to precisely construct structures in small scales, has emerged as considerable new tool to enhance the controllability of porous media, and furthermore, to achieve a good solution to minimize the plugging disadvantage. The present study selected a transpiration cooling configuration perforated by straight holes with an additive manufacturable diameter of 0.4 mm. Computational Fluid Dynamics (CFD) methods were employed to model the pore plugging and its effect on heat transfer. A scripting code in addition to the ANSYS CFX solver was utilized to simulate the random plugging conditions of the holes. Two hundred numerical cases with four different plugging probabilities were calculated and statistically evaluated to quantify the disadvantage of pore plugging on the cooling effectiveness. A theoretic model with convolution functions was developed to predict the local cooling effectiveness. Results obtained from the numerical analysis indicated that the overall plugging ratio was a dominating parameter for the cooling effectiveness but this single parameter was not adequate to scale the cooling effectiveness for all locations. On the contrary, the unique pair of discrete convolution parameters indexing all other transpiration holes in the array developed in this study had a significantly higher accuracy in predicting the cooling effectiveness than the overall plugging ratio. The present study was among one of the earliest to use convolution modeling method to predict transpiration cooling and related plugging disadvantages. This effort could provide a quantitative understanding of the random plugging on the specific transpiration cooling configuration, and could benefit further optimization effort to reduce the plugging disadvantage of transpiration cooling using additive manufacturing.

The significance of mass transfer enhancement in polymer electrolyte fuel cells (PEFC) is presented and studied in this work based on experimental investigation. A novel structure of reactant gas distributors in PEFC is proposed for mass transfer enhancement purpose. For the PEFC with novel gas distributors, it is found that the large drop of the cell voltage, generally caused by a weak mass diffusion, is postponed to occur at relatively higher current density even though the same or less amount of air is fed when comparing to a PEFC with gas distributors in conventional structure. As a result, the maximum obtained electrical power in a PEFC and a PEFC stack both are dramatically improved under both free convective and forced convective airflow conditions.

The heat and species transport processes in a tubular type solid oxide fuel cell (SOFC) that works in a cell stack were analyzed and modeled. Since most of the single tubular SOFCs working in a cell stack share the same/similar chemical/electrochemical and heat/mass transfer conditions, it is plausible to assume that heat and species are not exchanged between one cell and its neighboring cells. Therefore, a surrounding fuel flow space was outlined controllable by a specific single cell, for which zero flux was assumed at its boundary in neighborhood with other cells. The numerical model subjects such a cell and its controllable fuel flow space to a two-dimensional analysis for the flow, heat/mass transfer and chemical/electrochemical performance. Computations were performed for three different tubular SOFCs having practical operating results available from publications by different researchers. The numerical results of the terminal voltages for those different SOFCs showed very good agreement with the published experimental data. It is expectable that the proposed numerical model be used to significantly help the design and operation of a SOFC stack in practical applications.

Two types of miniaturized PEM fuel cells are designed and characterized in comparison with a compact commercial fuel cell device in this paper. One has Nafion® membrane electrolyte sandwiched by two brass bipolar plates with micromachined meander-like gas channels. The cross-sectional area of the gas flow channel is approximately 250 by 250 (μm). The other uses the same Nafion® membrane and anode structure, but in stead of the brass plate, a thin stainless steel plate with perforated round holes is used at cathode side. The new cathode structure is expected to allow oxygen (air) being supplied by free-convection mass transfer. The characteristic curves of the fuel cell devices are measured. The activation loss and ohmic loss of the fuel cells have been estimated using empirical equations. Critical issues such as flow arrangement, water removing and air feeding modes concerning the fuel cell performance are investigated in this research. The experimental results demonstrate that the miniaturized fuel cell with free air convection mode is a simple and reliable way for fuel cell operation that could be employed in potential applications although the maximum achievable current density is less favorable due to limited mass transfer of oxygen (air). The relation between the fuel cell dimensions and the maximum achievable current density is also discussed with respect to free-convection mode of air feeding.

Abstract Effusion cooling has been recognized as the next generation of cooling technologies for gas turbine engines. Concurrent effusion cooling configurations generally consist of a large number of film cooling holes to form full coverage coolant films on the protected surfaces. Coolant superposition effect consequently became a dominating factor for effusion cooling, and increased the difficulty to correlate the effectiveness with geometric parameters. This study proposed a machine learning approach using the convolution modeling method to predict the local adiabatic cooling effectiveness for effusively cooled surfaces. The model was trained by the numerical simulation data of several regular film cooling hole arrays, then validated by the data of randomly distributed film cooling hole rows. This model utilized convolution calculations in the regression process and successfully reconstructed the cooling effectiveness distribution for the entire surfaces. Results showed a good accuracy for both the training group and the validation group. Additionally, the convolution kernel of the model visualized the effect of coolant superposition by quantifying the contribution of a neighbor coolant ejection to the target location. This machine learning approach could serve as a strong tool to regress the adiabatic cooling effectiveness effusion cooling. Additional capability of this method could be exploited in other film cooling sections in turbine engines, including combustors liners, turbine blades and turbine endwall.

In this study, the thermoelectric performance of an integrated thermoelectric device (iTED) with rectangular, round end slots, and circular flow channel designs applied to waste heat recovery for several hot stream flow rates has been investigated using numerical methods. An iTED is constructed with p- and n-type semiconductor materials bonded to the surfaces of an interconnector with flow channels drilled through it. This interconnector acts as an internal heat exchanger directing waste heat from the hot stream to thermoelectric elements. The quantity of heat extracted from the waste heat source and the subsequent amount of electrical power generated P0 from the iTED is increased significantly for the circular flow channels, followed by round end slots and rectangular flow channels, respectively. At Re = 100, the round end slots and the circular flow channels showed nearly 2.6 and 2.9 times increment in P0, and 1.5 and 1.65 times in when compared to the rectangular flow channels values. Conversely, when Re is increased from 100 to 500, the iTED with rectangular flow channels showed 2.67- and 1.6-fold improvement in P0 and , respectively. However, the circular configurations showed 2.27- and 1.41-fold increases in P0 and values, respectively. Within theRe range studied, the inclusion of flow channels’ pumping power in calculations showed negligible effect. For an iTED with circular flow channels, an increase in a cold side convective heat transfer coefficienthc resulted in an enhancement inP0 and values. Besides a hc effect, the heat loss to the ambient via convective and radiation heat transfer exhibited an increase inP0 and decrease in .

The current detailed experimental study focuses on the optimization of heat transfer performance through jet impingement by varying the coolant flow rate to each individual jet. The test section consists of an array of jets, each jet individually fed and metered separately, that expel coolant into the channel and exit through one end. The diameter D, height-to-diameter H/D, and jet spacing-to-diameter S/D are all held constant at 9.53 mm, 2, and 4, respectively. Upon defining the optimum flow rate for each jet, varying diameter jet plates are designed and tested using a similar test setup with the addition of a plenum. Two test cases are conducted by varying the jet diameter within 10% compared to the benchmark jet diameter, 9.53 mm. The Reynolds number, which is based on hydraulic diameter of the channel and total mass flow rate entering the channel, ranges from approximately 52,000 up to 78,000. The transient liquid crystal technique is employed in this study to determine the local and average heat transfer coefficient distributions on the target plate. Commercially available computational fluid dynamics software, ansys cfx, is used to qualitatively correlate the experimental results and to fully understand the flow field distributions within the channel. The results revealed that varying the jet flow rates, total flow varied by approximately ±5% from that of the baseline case, the heat transfer enhancement on the target surface is enhanced up to approximately 35%. However, when transitioning to the varying diameter jet plate, this significant enhancement is suppressed due to the nature of flow distribution from the plenum, combined with the complicated crossflow effects.

Abstract A three-dimensional (3D) numerical model associating the heat/mass transfer and the electrochemical reaction in a proton exchange membrane (PEM) fuel cell is developed in this study, and a miniaturized PEM fuel cell with complex flow channels is simulated. The numerical computation is based on the finite-volume method. Governing equations for flow and heat/mass transfer are coupled with the electrochemical reactions and are solved simultaneously. The latent heat from the condensation of water vapor in cathode channel, if any, is considered. The perimeters of the bipolar plates are also included in the computational domain to account for their heat conduction effect. The miniaturized PEM fuel cell has a membrane electrode assembly (MEA) sandwiched by two brass bipolar plates etched with a number of winding gas channels with a flow area of 250 μm ×250 μm. The influence of anode gas humidity on the performance of the fuel cell is investigated through model prediction. Finally, field details of velocity, mass fraction and electromotive force are illustrated and discussed.

Peer-reviewed journal article Applied Rheology