Jet Impingement Heat Transfer Characteristics with Variable Extended Jet Holes under Strong Crossflow Conditions

In this paper, detailed flow patterns and heat transfer characteristics of a jet impingement system with extended jet holes are experimentally and numerically studied. The jet holes in the jet plate present an inline array of 16 × 5 rows in the streamwise (i.e., the crossflow direction) and spanwise directions, where the streamwise and spanwise distances between adjacent holes, which are normalized by the jet hole diameter (xn/d and yn/d), are 8 and 5, respectively. The jets impinge onto a smooth target plate with a normalized distance (zn/d) of 3.5 apart from the jet plate. The jet holes are extended by inserting stainless tubes throughout the jet holes and the extended lengths are varied in a range of 1.0d–2.5d, depending on the jet position in the streamwise direction. The experimental data is obtained by using the transient thermochromic liquid crystal (TLC) technique for wide operating jet Reynolds numbers of (1.0 × 104)–(3.0 × 104). The numerical simulations are well-validated using the experimental data and provide further insight into the flow physics within the jet impingement system. Comparisons with a traditional baseline jet impingement scheme show that the extended jet holes generate much higher local heat transfer levels and provide more uniform heat transfer distributions over the target plate, resulting in the highest improvement of approximately 36% in the Nusselt number. Although the extended jet hole configuration requires a higher pumping power to drive the flow through the impingement system, the gain of heat transfer prevails over the penalty of flow losses. At the same pumping power consumption, the extended jet hole design also has more than 10% higher heat transfer than the baseline scheme.

Thermal Resistance and Pressure Drop Minimization for a Micro-gap Heat Sink with Internal Micro-fins by Parametric Optimization of Operating Conditions

In recent years, researchers are investigating several potential applications of two-phase flow in micro-gap heat sinks; electronic cooling is one of them. Further, internal micro-fins are used to enhance the heat transfer rate. However, the pressure drop penalty due to small gap height and fin surfaces is a major concern. Hence, minimization of thermal resistance and pressure drop is required. In this paper, effects of operating conditions, e.g., wall heat flux, pumping power, and inlet void fraction, on total thermal resistance and pressure drop in a micro-gap heat sink with internal micro-fins of rectangular and triangular profiles have been investigated by numerical analysis for the R-134a coolant. Furthermore, optimization of these parameters has been carried out by response surface methodology. Simulation results show that rectangular micro-fins show superior performance compared to triangular fins in reducing thermal resistance. Finally, for an optimum condition (7.1202×10-5 W pumping power, 1.2×107 Wm-2 heat flux, and 0.03 inlet void fraction), thermal resistance and pressure drop are reduced by 56.3% and 87.2%, respectively.

Heat transfer enhancement using double taper microchannel

A three-dimensional numerical study on the combined effect of height as well as width tapering on the thermal performance of double taper microchannel is presented in this paper. The channel inlet width is considered as 300 µm, taper ratio on sidewalls and bottom wall are varied from 0 to 1 and 1 to 3.9, respectively. The thermal resistance ratio, average bottom wall temperature, temperature difference ratio, and pumping power ratio of the channel are evaluated for various flow rates, height, and width tapering. Results showed higher reduction of wall temperature with combined effect height as well as width tapering compared with straight channel. The optimal size of the micro channel to minimize the pumping power and average wall temperature on the constraint of heat flux and footprint area is found. The reduction in average bottom wall temperature is 17.34%, and pumping power ratio is 0.44 (56% power reduction) noted, respectively, at Reynolds number 340. Finally, optimal dimension of double taper microchannel is evaluated for better thermo-hydraulic performance.

Investigation of SiGeSn/GeSn/SiGeSn single quantum well with enhanced well emission

In this work, a SiGeSn/GeSn/SiGeSn single quantum well was grown and characterized. The sample has a thicker GeSn well of 22 nm compared to our previously reported 9-nm well configuration. The thicker well leads to: i) lowered ground energy level in Γ valley offering more bandgap directness; ii) increased carrier density in the well; and iii) improved carrier collection due to increased barrier height. As a result, significantly enhanced emission from the quantum well was observed. The strong photoluminescence signal allows for the estimation of quantum efficiency, which was unattainable in previous studies. Using pumping-power-dependent photoluminescence spectra at 20 K, the peak spontaneous quantum efficiency and external quantum efficiency were measured as 37.9% and 1.45%, respectively.

Performance Analysis of Single-Phase Space Thermal Radiators and Optimization through Taguchi-Neuro-Genetic Approach

In the thermal management of spacecraft, space thermal radiators play a vital role as heat sinks. A serial radiator with proven advantages in ground applications is proposed and analyzed for space applications. From the performance analysis, specific heat rejection of serial radiator is found to be higher than parallel radiator by 80% for maximum diameter of tube, 47% for maximum thickness of fin, and 75% for maximum pitch of tubes under consideration. Also, serial radiator requires four times higher pumping power than parallel radiator with geometric parameters and a maximum mass flow rate under consideration. In serial radiators, the cross conduction between the fins has a significant effect on its thermal performance. Thus, conjugate heat transfer simulations and optimization operations are to be performed iteratively to optimize the serial radiator, which is computationally costly. To reduce the computational time, Artificial Neural Network is trained using conjugate heat transfer simulations data and combined with the genetic algorithm to perform optimization. Taguchi's orthogonal arrays provided the partial fraction of conjugate heat transfer simulations set to train the ANN. Taguchi-Neuro-Genetic approach, a process that combines the features of three powerful techniques in different optimization phases, is used to optimize both parallel and serial radiators. The optimization aims to obtain a configuration that provides the lowest mass and lowest pumping power requirement for given heat rejection. Optimization results show that the conventional parallel radiator is about 20% heavier and requires about 35% more pumping power than the proposed serial radiator.

Competing processes in generation of the third optical harmonic in air under femtosecond infrared repetitively pulsed excitation

Parametric interaction of electromagnetic and gravitational waves with the radiation generation at the third harmonic wavelength is one of the ways to detect gravitational interaction in a material medium. To implement the effect in question, superstrong fields must be used, but in this case competing nonlinear processes arise, leading to the generation of the third harmonic as a result of laser radiation filamentation. This paper investigates the characteristics of the radiation recorded for femtosecond (250 fs) laser pulses with a wavelength of λ = 1032 nm focused in air. The threshold pump power made it possible to observe the formation of a filament with concomitant generation of narrow-band radiation at the focus of the lens at the third harmonic wavelength λ = 344 nm. The research presents spectral and spatial dependences of ultraviolet radiation (λ = 344 nm) at pumping power of infrared radiation (λ = 1032 nm) of 500 mW. Energy dependences of the third harmonic generation efficiency in the power range from 150 to 1750 mW are obtained.

Microchannel Heat Sink for Thermal Management of Concentrated Photovoltaic Cells

This article conceptualizes a single-phase microchannel heat sink for thermal management of concentrated photovoltaic cells; details of the model-based parametric study that is carried out on the heat sink is also detailed in this article. The heat sink consists of multiple serpentine microchannels. The mathematical model consists of continuity equation, Navier-Stokes equations and energy equations. Fluent module of Ansys Workbench is used for solving the model. The performance of the device is quantified in terms two metrics such as thermal resistance and pumping power. Studies are done for Reynolds number ranging from 100 to 1250. It is observed that increase in Reynolds number decreases the thermal resistance while increasing the pumping power irrespective of the geometric parameters of the heat sink. Decrease in hydraulic diameter of the microchannel reduces the thermal resistance while increasing the pumping power. Increase in the length segment of the serpentine microchannel increases and decreases the thermal resistance and pumping power, respectively. With increase in the offset width of the serpentine microchannel the thermal resistance and pumping power decreases and increases, respectively.

The Optimal Pumping Power under Different Ice Slurry Concentrations Using Evolutionary Strategy Algorithms

A suitable ice slurry fluid with a suitable ice concentration ratio can save operational costs. The design of the optimal ice slurry concentration focuses on finding an evolution strategy, which can further minimize the power consumption of the pump. A theoretical model was established to simulate the effect of different ice concentrations and flow rates on the performance of the pump. The data obtained were fitted by curve-fitting function. The process was modeled in the MATLAB evolutionary strategy algorithm to obtain the configuration scheme of the ice concentration and flow under different refrigeration capacities. The simulation results showed that when the required cooling capacity was 13.889 kWh, ice concentration was set to 19.68%, and flow rate was set to 2.1075 × 10−4 m3/s, the power consumption could be reduced by 23%.

Using Graphene Nanoplatelets Nanofluid in a Closed-Loop Evacuated Tube Solar Collector—Energy and Exergy Analysis

Recently, nanofluid application as a heat transfer fluid for a closed-loop solar heat collector is receiving great attention among the scientific community due to better performance. The performance of solar systems can be assessed effectively with the exergy method. The present study deals with the thermodynamic performance of the second law analysis using graphene nanoplatelets nanofluids. Second law analysis is the main tool for explaining the exergy output of thermodynamic and energy systems. The performance of the closed-loop system in terms of energy and exergy was determined by analyzing the outcome of field tests in tropical weather conditions. Moreover, three parameters of entropy generation, pumping power and Bejan number were also determined. The flowrates of 0.5, 1 and 1.5 L/min and GNP mass percentage of 0.025, 0.5, 0.075 and 0.1 wt% were used for these tests. The results showed that in a flow rate of 1.5 L/min and a concentration of 0.1 wt%, exergy and thermal efficiencies were increased to about 85.5 and 90.7%, respectively. It also found that entropy generation reduced when increasing the nanofluid concentration. The Bejan number surges up when increasing the concentration, while this number decreases with the enhancement of the volumetric flow rate. The pumping power of the nanofluid-operated system for a 0.1 wt% particle concentration at 0.5 L/min indicated 5.8% more than when pure water was used as the heat transfer fluid. Finally, this investigation reveals the perfect conditions that operate closest to the reversible limit and helps the system make the best improvement.