Heat Transfer最新文献

The use of fossil fuel and wood for cooking poses health, environmental, and economic challenges, especially with the growing population. This has led to an increase in the trend towards the use of clean and sustainable cooking sources, including solar cookers. This experimental study aims to contribute by enhancing the performance of a solar box cooker (SBC) according to the concept of porous media via adding steel fibers inside the box as a modified SBC and comparing it with a conventional SBC. The stagnation test to determine the first figure of merit and the load test to determine the second figure of merit, standard boiling time, and cooker optothermal ratio were conducted under the outdoor conditions of Baghdad city. Also, an energy and exergy efficiency analysis, and calculating the rate of heat loss by three water loads heating and cooling tests. The results revealed that the modified SBC has a higher thermal performance than the conventional SBC.

The thermal performance of the flat plate solar collector is very low. The most beneficial and worthwhile method for increasing the thermal performance of a solar-powered air heater (SPAH) is to include a roughness element in the working zone of heat transfer that is located beneath the shear layer of the absorber surface. In this research work, efforts are made to enhance thermal performance and develop thermal correlations for the estimation of the Nusselt number and friction factor of a roughened SPAH. Experiments are performed for various ranges of flow, Reynolds numbers, and roughness parameters. The experimental technique of liquid crystal thermography is utilized to assess the dispersal of Nusselt number over the roughened surface for all roughness parameters. A maximum thermal performance enhancement index of 2.69 is obtained with the optimum value of the roughness parameter at a relative roughness pitch (RRP) of 9, a relative staggering distance (RSD) of 4, and a relative roughness length (RRL) of 6.15. Second, a mathematical correlation is developed using a regression model to estimate the Nusselt number and friction factor in terms of nondimensional roughness and flow parameters operated as RRP, RSD, RRL, and Re. The degree of discrepancy between the established the relationships and the findings from the experiment reveals incredibly satisfying results. Hence employing twisted V-ribs as an artificial roughness element no doubt increases the Nusselt number, and thermohydraulic performance enhancement index, but it also exerts less frictional power across the SPAH duct.

This article aims to achieve exact and analytical solutions for the classical Falkner–Skan equation with heat transfer (FSE-HT). Specifically, when the pressure gradient parameter $��\beta �\; �=�\; �-�\; �1�$, there already exists a closed-form solution in the literature for the Falkner–Skan flow equation. The main purpose here is to extend this case to obtain a closed-form solution to the heat transport equation with the solubility condition $��P�\; �r�\; �=�\; �1�$. An algorithm is presented and is found to be new to the literature that enriches the physical properties of FSE-HT. It is shown that for the moving wedge parameter $��\lambda �\; �>�\; �1�$, the momentum and temperature equations show multiple solutions analytically. The skin friction coefficient and the heat transfer rate are also obtained in analytical form. The thus-obtained solution is then adapted to derive an analytical solution applicable to a wide range of pressure gradient parameters $��\beta �$ and Prandtl numbers $��P�\; �r�$. Furthermore, an asymptotic analysis is conducted, focusing on scenarios where the moving wedge parameter becomes significantly large ($��\lambda �\; �\to �\; �\infty �$). Nevertheless, in all the above-mentioned cases, the skin friction coefficient ($��f�\; �$

The impact of the double-diffusive natural convection of water in a square enclosure with diffusion-thermo and thermo-diffusion around the non-Darcy porous medium is studied numerically. The top and bottom side walls of the cavity are maintained at constant temperatures, while the vertical left and right walls are considered to be cold. Water is considered the working fluid. The resulting nondimensional partial differential equation is solved by the Marker and Cell (MAC) method by using a staggered grid system. The Dufour and Soret effects processes will be a significantly essential role in the double-diffusive velocity flow processes. For different values of the relevant parameters, the fluid flow velocity, temperature, and concentration are presented for various Rayleigh numbers, Prandtl numbers, non-Darcy, thermo-diffusion, and diffusion-thermo. The streamlines, isotherms, and iso-concentration contours are obtained using the MATLAB software. Finally, it is observed that the Nusselt numbers increased with non-Darcy, thermo-diffusion, and Rayleigh numbers and decreased with diffusion-thermo effects.

A hybrid genetic algorithm (GA)–Kohonen map, with its three variants, is explored for the first time for the decision-making system in a porous ceramic matrix (PCM)-based burner through determination of the regime of operation. Four different attributes of PCMs such as convective coupling (P₂), extinction coefficient (β), downstream porosity (ϕ₂), and scattering albedo (ω) are selected for determining the regime of operation of a PCM-based burner. Changes in any of these attributes of a PCM lead to significant changes in the temperature profiles of the gas and solid phases. Temperature profiles of the gas and solid phases are computed by developing a numerical model. Various samples corresponding to different regimes are generated and used in a hybrid GA–Kohonen map. The best architectural details such as the neuron number and training epochs are obtained from GA as output. The best Kohonen map is trained with the input data, and regimes of operation for new temperature profiles are predicted. A supervised Kohonen map is able to provide the highest average class prediction of more than 40%. All the variants are assessed under two different types of neuron grids: hexagonal and rectangular. Comparative assessments of the three different variants of Kohonen maps, in terms of CPU time and average class prediction, are carried out.

The current research investigates the impact of a split fueling strategy combined with several flow rates of exhaust gas recirculation (EGR) on the combustion and emission characteristics of a diesel engine running on B20 waste cooking oil (WCO) biodiesel. A four-stroke single-cylinder common rail direct injection engine was employed for experiments. It operates with a B20 blend of WCO biodiesel at 600 bar pressure for varying pilot fueling conditions of 10%, 20%, and 30%. The B20 blend with 30% pilot fuel injection (B20P30) showed excellent performance and emission characteristics compared with B20 blend with 10% pilot fuel injection (B20P10) and B20 with 20% pilot fuel injection (B20P20). However, B20P30 had greater levels of nitrogen oxide (NO_x) emissions than those by diesel. EGR discharge levels in 5% increments, ranging from 0% to 15% were introduced to address this issue. The experimental findings revealed that both cylinder peak pressure and heat release rate showed a reduction when the EGR flow rate was enhanced. The recirculation of exhaust gas into the combustion chamber led to a slight increase in the emission levels of hydrocarbon (HC), carbon monoxide (CO), and smoke, as well as a decrease in carbon dioxide (CO₂). Nevertheless, the introduction of EGR significantly decreased NO_x emissions by 22.94%, 35.05%, and 47.96% for EGR flow rates of 5%, 10%, and 15%, respectively, when compared with the engine operating without EGR. Overall, the two-stage fueling strategy, B20P30 blended with 10% EGR corroborated to be beneficial in reducing NO_x emissions with minimal performance penalties. Although there was a slight uptick in certain emissions, the overall trade-off between emission reduction and performance was favorable. The culmination of this study is targeting the objectives of sustainable development goal 7 (clean energy) and goal 13 (climate action) to be achieved by 2030.

Ground-coupled heat exchangers (GCHE) have received significant attention over several decades as a result of increasing the world's energy demand and the need for reducing fossil fuels consumption. Prior studies have demonstrated the effectiveness of utilizing GCHE with refrigeration and heating systems. However, optimizing the performance of GCHE coupled with chillers for heat rejection, especially in extreme hot-humid climates (where cooling towers are not very effective) is lacking in the literature. In this work, a ground borehole fitted with a coaxial-tubes heat exchanger (BHE) is numerically simulated. Based on a wide range of data collected for the soil of Dubai, its real in situ thermophysical properties are characterized. The soil's upper layer thickness is relatively small and dry that operates in conduction mode, while the lower one is water-saturated that works in coupled conduction-advection mode. The study aims at optimizing the parameters advancing heat rejection into ground considering the actual properties of the soil of Dubai. The results indicate that the more feasible high-density polyethylene pipes can perform as good as the steel ones. Also, a great finding based on the presented novel design is that insulating the inner pipe can increase the temperature duty by 55%. The proposed design of BHE is relatively inexpensive, more feasible and efficient, which is achieved for the first time based on a deep analysis of Dubai climate and soil. This makes the technology ready to be implemented for industrial applications in Dubai and other regions having a similar climate and soil nature.

The thermohydraulic performance (THP) of a solar air heater (SAH) duct with staggered D-shaped ribs as roughness geometry is examined in this work using three-dimensional numerical investigation. The investigation is carried out at roughness parameters of radius of rib to transverse pitch (r/P_tv) ratios of 0.1–0.35 and longitudinal pitch to radius of rib (P_lg/r) ratios of 4–10 under varied operating circumstances of Reynolds number (Re) from 10,200 to 20,200. The maximum Nusselt number (Nu) is obtained to be 81.3 at Re of 20,200, r/P_tv as 0.1, and P_lg/r as 4. In contrast, the maximum friction factor (f) is obtained to be 0.0169 at Re of 10,200, r/P_tv as 0.35, and P_lg/r as 4. In the range of parameters examined, the maximum enhancement in Nusselt number (Nu/Nu_s) is observed to be 1.35 at an optimal parameter of r/P_tv as 0.1, P_lg/r as 4, and Re as 10,200. Correspondingly, the enhancement in the friction factor (f/f_s) at this optimum parameter is 1.87. The maximum value of the THP parameter is found to be 1.1 at the same optimum range of parameters. In further analysis, correlations were developed for Nusselt number (Nu) and friction factor (f) in terms of r/P_tv, P_lg/r, and Re with a deviation of ±2% and ±1.5%, respectively.

Analyzing fluid dynamics and heat transfer holds significant importance in the design and enhancement of engineering systems. The current investigation utilizes the finite element method to explore natural convection and heat transfer intricacies within a novel cavity containing an inner circular cylinder under steady and laminar flow conditions. The principal aim of this study is to assess the impact of Rayleigh number (Ra), Bejan number (Be), and the presence of adiabatic, hot, and cold cylinders on heat transfer, entropy generation, and fluid flow. The range of Ra considered in this investigation spans from 10³ to 10⁶, while the Prandtl number for the air is fixed at 0.71. The findings illustrate that the presence of a cylinder leads to higher Be as Ra increase, compared to scenarios where no cylinder is present. This observation suggests that buoyancy forces dominate in the absence of a cylinder, resulting in significantly enhanced convective heat transfer efficiency. However, the presence of a heated cylinder within the tooth-shaped cavity exerts a substantial influence on the overall thermal performance of the system. Notably, the average Nusselt Number (Nu) experiences a remarkable increase of 41.97% under the influence of a heated cylinder, when compared to situations where a cold cylinder is present. This elevated average Nu signifies improved heat transfer characteristics, ultimately resulting in an overall improvement in the thermal system's efficiency.

Falkner's-Skan flows are one-dimensional flows widely used in fluid dynamics and boundary layer theory. They optimize heat exchangers and cooling systems, enhance turbine and compressor blades in turbomachinery, and provide consistency and quality in semiconductor fabrication. Additionally, they assist in pollution management and environmental impact assessments. The variety of Falkner's-Skan flows clarifies fluid flow phenomena and their practical applications. This article aims to explore the impact of surface temperature on the flow behavior of dusty fluid in Falkner's-Skan flow. The study focuses on the flow of dust particles in a channel formed by two infinite parallel plates. The study assumes that the particles are round and uniformly dispersed in the fluid. Furthermore, the article takes into account the effect of radiation on the energy equation. With the findings of this study, we hope to gain a better understanding of the dynamics of Falkner's-Skan flow and contribute to the development of effective strategies for managing the flow of dusty fluids. The right plate's movement at free stream velocity $��U_e�\; �=�\; �a�\; �x^n�$ causes the fluid to flow. Partial differential equations are used to represent the behavior of the flow. The Poincare-Light Hill Technique yields exact answers. Visual representations of the temperature and velocity curves show the effects of different factors. It is possible to create graphic pictures of the fluid and dust particles using Mathcad-15. Furthermore, critical fluid characteristics for engineers, such as skin friction and heat transfer rate, are analyzed and tabulated. These evaluations include the heat transfer rate at the wedge surface and the influence of this enhancement on surface viscous drag forces.