Abstract As a novel, compact, and efficient plate-fin heat exchanger, the Printed Circuit Heat Exchanger (PCHE) is a prospective candidate for liquefied natural gas (LNG) vaporization at low-temperature and high pressure. Generally, the airfoil fin PCHE has better thermal-hydraulic performance than the zigzag channel PCHE. In this study, the thermal-hydraulic performance of supercritical LNG in PCHEs with different airfoil fin types and arrangements is investigated by numerical simulations. First, the effects of six different airfoil fin types, NACA0010, NACA0020, NACA0025, NACA0030, NACA 0040, and NACA 0050, on the thermal-hydraulic performances were studied. The results show that NACA0025 has the best comprehensive heat transfer performance. Then, the effects of staggered, vertical, and horizontal pitch of the airfoil fin arrangement on thermal-hydraulic performance were investigated. The results show that the optimal values of the dimensionless number for staggered and vertical arrangements are 1 and 4, respectively. The comprehensive performance does not change much when the dimensionless horizontal pitch number exceeds 3.0. Finally, the thermal-hydraulic performance of uniformly distributed, three front sparse and rear dense, and three front dense and rear sparse distributed airfoil fins was investigated. The results show that the front dense and rear sparse airfoil fins enhance and the front sparse and rear dense airfoil fins reduce the comprehensive performance compared to the uniform arrangement. The results show that a denser arrangement of airfoil fins near the quasi-critical point can improve the comprehensive performance while keeping the number of airfoil fins constant.

Abstract The thermal characteristics of a variable temperature, flowing vapor cryostat are theoretically modeled, taking into account specific geometrical and material constraints, temperature-varying heat transfer coefficients, and thermal conductivities for conductive, convective, and radiative heat transfer. The temperature within the cryostat is controlled by an internal heater and is monitored at both the heater and the sample stage. The modeled system consists of multiple coaxial, cylindrical layers of stainless steel containing various fluids (light vacuum, helium gas, nitrogen gas; the liquid cryogen is nitrogen or helium). The calculated Prandtl and Grashof numbers for the fluid layers suggest that the Churchill-Chu form of the Nusselt equation be used for heat transfer analysis of this system. Developing a model that predicts heat flows throughout the cryostat allows for appropriate articulation of the heater so that the sample quickly reaches the desired temperature without overshooting. Transient and steady-state models are investigated for predictive ability and consistency with the system's experimentally collected heating and cooling behavior.

Abstract This paper proposes a new ORC-VCC(Organic Rankine Cycle+Vapor compression cycle) system(with mechanical overheating refrigeration cycle), and this system can not only reduce the heat absorption of the ORC evaporator, but also increase the refrigeration capacity of the system. Simulations were conducted to analyze the thermal efficiency and performance of the new system, and compare it with the system of ORC-VCC(with regenerator). The results show that the ηth, ηsys and COPsys(coefficient of performance) of new system are higher than the system of ORC-VCC(with regenerator), ηth, ηsys and COPsys of new system increased by up to 31.6%, 6.48%, 10.63% respectively. And the influence of superheat on both systems is stronger than other factors, the influence of superheat on the new system is obviously stronger than those of the system of ORC-VCC(with regenerator), and the influence of superheat on R245fa and Butane is stronger than those of other working fluids. In addition, ηth, ηsys, COPsys and ηex of system increase with the increase of Te-mech and decrease with the increase of Tg-mech. Finally, the correlation of δTmax with the change of ηexp and Te-orc is fitted, the results will provide some reference for the development of the ORC-VCC system in the future.

Abstract This research aims to create an artificial neural network (ANN) regression model for predicting the performance parameters of the perforated micro-pin fin (MPF) heat sinks for various geometric parameters and inflow conditions. A three-dimensional computational fluid dynamics (CFD) simulation system is developed to generate dataset samples under different operational conditions, which are specified using Latin hypercube sampling (LHS). An ANN model is first obtained by optimizing the model hyper-parameters, which are then deployed to learn from the input feature space that consists of perforation diameter, perforation location, and inflow velocity. For accurate training of the ANN, the model is trained over a range of uniformly distributed data points in the input feature space. The developed multi-layer model predicted Nusselt number and friction factor with the mean absolute percentage error of 4.45% and 1.80%, respectively. Subsequently, the developed surrogate model is used in the optimization study to demonstrate the application of the surrogate model. A multiobjective non-dominated sorting genetic algorithm (NSGA-II) is used to perform the optimization of the perforation location, diameter, and inflow conditions. Negative of the Nusselt number and friction factor are chosen as objectives to minimize. A Pareto front is obtained from the optimization study that shows a set of optimal solutions. Thermal performance of the perforated MPF is increased between11.5% to39.77%. The optimizer selected a significantly smaller hole diameter at a higher location and a faster speed to maximize the Nusselt number and minimize the friction factor.

Abstract Gas turbine power plants play a crucial role in meeting the growing demand for electrical energy. However, their performance can be hindered by high ambient temperatures and humidity levels in tropical climates, leading to a drop in power output. This study investigates the potential benefits of using inlet air cooling with desiccant dehumidification and evaporative cooling to improve the performance of gas turbine power plants in tropical regions. The results show that this inlet air cooling method, integrating evaporative cooling, desiccant wheel, and Maisotsenko cooler, is a viable alternative for mitigating the performance decrease of gas turbines in hot tropical conditions. Furthermore, the compressor inlet temperature can be reduced on average by 11.5 °C by using turbine exhaust gases to heat the regeneration air utilized in the desiccant wheel for dehumidification. Additionally, the power requirement of the inlet air cooling system amounts to around 0.9 MW compared with an improvement of more than 2 MW in power output at peak temperature. Further research is needed to understand and quantify other benefits related to inlet air cooling, such as reducing emissions of harmful pollutants and operating at higher turbine inlet temperatures.

Abstract In order to increase biogas production during winter months/cold climatic conditions, a self-sustained N-photo-voltaic thermal and flat plate collectors (PVT-FPC) hybrid active heating biogas plant has been analyzed in terms of thermal energy and electrical energy. A general analytical expression for thermal and electrical energy of biogas plant has been derived as a function of climatic and design parameters from one order coupled differential equations. Various photo-voltaic thermal and flat plate collectors (N-PVT-FPC) configurations have been considered for optimizing maximum electrical and thermal energy gain for a given total number of N-PVT-FPC collectors. Based on mathematical computation for Indian cold climatic conditions, it has been found that the photo-voltaic thermal and flat plate collectors (N-PVT-FPC) combination is always better than the flat plate collectors-photo-voltaic thermal (N-FPC-PVT) collector for maximum electrical and thermal energy. Overall exergy analysis of hybrid active heating of biogas plant has also been carried out.

Abstract The labyrinth seal is effective in reducing leakage losses at the rotor blade top in the turbine. This study investigates the variation in labyrinth seal performance at different speeds, different Reynolds numbers and different tooth front angles. Three Reynolds numbers (, 10000, 15000), five rotational speeds (, 0.01, 0.04, 0.08, and 0.1), and three tooth front angles(75°, 90°, and 102.4°) have been introduced. The variation of leakage losses and heat transfer under different conditions is compared and a detailed analysis of the flow field and energy losses is performed. The discharge coefficient is increased slightly with increased rotational speed for the same Reynolds number. This is caused by the high rotational speed reducing the throttling loss and vortex loss. The high speed enhances the heat transfer at the tip wall of the passage, but also weakens the heat transfer at the tooth cavity bottom. Additionally, the sealing capacity of the labyrinth is better at large tooth front angles, which is caused by the reduction of frictional losses on the stator and eddy current losses in the tooth cavity. The change in local pressure loss also affects the velocity distribution along the channel, which is the reason for the change in the local Nusselt number.

Abstract Transformer-oil with low thermal conductivity and large viscosity has poor heat dissipation capability, which leads to the thermal drive failure caused by transient overload. To improve its cooling capability, this paper has proposed firstly the method combined the periodically direction-switching electric field and graphene nanofluid to enhance the mixed convective heat transfer properties of transformer-oil, and analyzed the effects of switching periods, nanofluid concentration, electric field strength, heat flux and Reynolds number on mixed convection heat transfer experimentally. The results show that the heat transfer characteristic of transformer-oil is improved up to 52% by the periodically direction-switching electric field and graphene nanofluid. As the switching period decreases, the thermal performance of the suspension is enhanced more significantly. Moreover, by analyzing the heat transfer mechanism, the periodically direction-switching electric field causes the nanoparticles to move reciprocally, repeatedly impacting and breaking the boundary layer of the heat exchange surface to enhance the perturbation, thus enhancing the heat transfer effect. Meanwhile, the predicted correlation has been proposed on the basis of influence factors, which are in good agreement with the experimental data.

Abstract Liquid lithium shows great promise as a coolant for the next generation of space nuclear reactors, and spiral tubes are commonly used in heat exchange devices. However, the heat transfer characteristics of liquid lithium in spiral tubes are not yet fully understood. This study develops a non-isothermal heat transfer model with a modified turbulent Prandtl number for liquid lithium flowing through spiral tubes with different geometries. Numerical analysis is carried out focusing on the influence of inlet velocity, the distribution of related parameters, and the geometry of spiral tubes. The results demonstrate that in the range of the dimensionless Dean number 8165-13063, the Nusselt number and the pressure drop present approximately linear relations with the Dean number. For the distribution law of relevant physical quantities, the inner side of the tube displays a kidney-shaped low-flow-rate area and a high-temperature area, while a low-pressure area forms on the inner pipe wall. Finally, the pitch and spiral radius are found to be reduced as much as possible to ensure high liquid lithium-based heat transfer performance with a small pressure drop. The optimized design parameters suggest 80-320 mm pitch and 120 mm spiral radius in the practical design scopes of 80-1280 mm and 120-360 mm, respectively. This work provides guidance for the heat transfer characteristics of liquid lithium in spiral tube-type heat exchangers, promoting its application in space nuclear reactor power supply.

Abstract In this paper, the flow condensation heat transfer characteristics of the environmentally friendly nearly-azeotropic refrigerant R1234ze(E)/R152a (mass ratio of 40:60) in smooth tubes with varying structures were numerically investigated. Under the operating conditions of mass flux of 400 kg · m−2 · s−1, heat flux of 12 kW · m-2, and saturation temperature of 308.15 K, this study investigated the influence of circular tube inner diameter, elliptical tube aspect ratio, and installation orientation on condensation heat transfer. The results indicate that the condensation heat transfer coefficient in the tube increases as the inner diameter of the circular tube decreases. The condensation heat transfer coefficient increases by 1.086 times when the circular tube diameter is reduced from 10.7 mm to 5 mm. Under identical operating conditions, the condensation heat transfer coefficient of the mixed refrigerant in elliptical tubes increases with an increase in the aspect ratio. The average condensation heat transfer coefficient increases by 18.21% as the aspect ratio of the elliptical tube increases from 1 to 2. Compared to a vertical elliptical tube, a horizontal elliptical tube is more favorable for condensation heat transfer within the tube.