查看更多>>摘要：This study presents a theoretical and numerical thermodynamic model for a beta -type Stirling engine with a rhombic-drive mechanism. This study focuses on developing a thermal model that overcomes the poor perfor-mance of previous models at high engine speed (above 60 Hz). The developed model is a modified version of the recently developed simple II model. The inaccuracy of the simple II model especially at high speed is improved by considering the power loss due to inertial forces and the variations of the gas temperature inside the heater and cooler with engine speed. The effect of several geometric and operational parameters. The accuracy of the developed numerical model is evaluated against experimental data of the GPU-3 Stirling engine. The difference between the current model predictions and experimental measurements for the output power and efficiency are 4.3% and 3.26% (as a difference), respectively. Moreover, the current model shows superior performance at high speed compared to previous models. It maintains its accuracy at high speed.
查看更多>>摘要：The use of the electrical analogy in heat transfer allows to simply solve many problems in various fields such as building or electrothermics. Nevertheless, it is limited to systems with two isotherms and a flux tube. The fin approximation allows to introduce an exchange on the flux tube, but the electrical analogy does not work anymore if there is a heat source inside the fin. In this paper, we will construct an electrical analogy for a system with: -On its boundary, several isotherms surfaces, a surface that exchanges with the external medium and a zero flux region, -In the volume, a heat source with any distribution. This approach generalizes the notions of thermal resistance and partition coefficient of the internal heat source. In the applications (here for a cylinder), the only approximation concerns the isotherms surfaces which in reality are not necessarily so (this approximation already exists in the elementary case, a thermal resistance is defined between two isotherms).
Hasbullah, NurjannahSaat, Fatimah Al Zahrah MohdAnuar, Fadhilah ShikhSukri, Mohamad Firdaus...
13页
查看更多>>摘要：Bi-directional flow condition imposes different fluid dynamics and temperature changes compared to that of the usual one-directional flow condition. Bi-directional flow can be found in applications like thermoacoustic systems that offer a green technology for at least two major applications: refrigeration and power production. The technology is appealing as an alternative to traditional systems as it offers the replacement for the use of harmful working media and exhausted resources with the use of inert gaseous with relatively fewer moving mechanisms. As the fluid dynamics and heat transfer of bi-directional flow in a thermoacoustic working environment is less known, it is difficult to estimate losses and gain, especially during the design stage. This paper reveals the differences to be expected in the behaviour of flow and heat transfer through experimental as well as Computational Fluid Dynamics (CFD) results of one-directional and bi-directional flow conditions. Two different drivers were used to create the two different flow conditions: a loudspeaker for the bi-directional flow and a centrifugal blower for the one-directional flow. Both conditions were monitored based on flow amplitude that is calibrated between the two drivers. Results of velocity, temperature and, vorticity are recorded for Reynolds number that ranges between 270 and 1700. Analyses are supplemented with data from validated two-dimensional computational fluid dynamics models that were solved using the Shear-Stress-Transport (SST) k-omega turbulence model with second-order accuracy for all equations. Interesting features of differences in temperature and velocity changes between the one-directional and the bi-directional flows are reported. The temperature and velocity at upstream and downstream locations of the tube banks heat exchanger are almost the same for bi-directional cases but are significantly different when a one-directional flow is flowing over the heated tubes. In addition, the interplay between natural and forced convections is seen to affect the results that were recorded for the two flow conditions. The presence of thermally developing and fully developed regions is also discussed. The results indicate that the heat transfer behaviour of bi-directional flow is not the same as in the one-directional flow and the future calculation for heat transfer for bi-directional flow conditions of thermoacoustic must be carefully done with consideration of changes of flow conditions between the one-directional and the bi-directional flow conditions so that error could be minimized in the evaluation of the system's performance.
查看更多>>摘要：An innovative way of advancing thermo-convection performance of thermal management systems is the application of binary nanofluids (BNFs) as thermal working fluids. This paper experimentally investigated for the first time the thermo-convection behaviour of MgO-ZnO nanoparticles dispersed in deionised water (DIW) for concentrations of 0.05 vol% and 0.1 vol% at percentage weight ratios (PWRs) of 20:80, 40:60, 60:40, 80:20 (MgO-ZnO) in a square cavity. Several parameters such as Ra, Nu(av), h(av), and Q(av) at different temperatures gradients (20 degrees C to 50 degrees C) were investigated. Viscosity and thermal conductivity of the BNFs and DIW were experimentally obtained for the studied temperatures. Temperature gradient and PWRs of hybrid nanoparticles in the BNFs were noticed to enhance Nu(av), h(av), and Q(av). In addition, maximum enhancement achieved were 72.6% (Nu(av)), 76.01% (h(av)), and 72.2% (Q(av)). The use of BNFs in a square cavity proved to yield good enhancement for thermoconvection performance. A new model linked to phi and PWRs has been introduced to predict Nu(av). The novel outcomes of this study further corroborate the synergetic advantage of using BNFs over NFs.
查看更多>>摘要：Effects of tangential heat conduction and moisture adsorption hysteresis of the membrane on the heat recovery performance of cross-flow and counter-flow membrane enthalpy exchangers are investigated numerically. The accuracy of the numerical simulation model is evaluated by predicting an experimental case and a reasonable accuracy is achieved. It is found that the temperature profile at the membrane surface is significantly influenced by the tangential heat conduction in the membrane. The variation of temperature profile at the membrane surface results in variations of the convective heat resistance and the sensible heat effectiveness. Simulation results demonstrate that the sensible heat effectiveness firstly increases and then decreases with the increasing membrane heat conductivity. The optimal membrane heat conductivity is about 0.1 ~ 1 W m(-1) K-1. Two types of the moisture adsorption hysteresis are assumed and expressed by simple expressions. It is found that the moisture adsorption hysteresis decreases the moisture diffusion flux across the membrane and accordingly decreases the latent heat effectiveness obviously for both counter-flow and cross-flow patterns. Current results indicate that membranes with large heat conductivity are not beneficial for sensible heat recovery and membranes with small adsorption hysteresis are beneficial for latent heat recovery.
查看更多>>摘要：This experimental work aims to study the effect of salts mixture solution in the metal quenching process. Discs of aluminum alloy AA6082 are heated up to 560 C and then exposed to a constant spray flux of 3 kg/m(2)s. This cooling process experiment uses an infrared camera to record the temperature history. Eight types of actual water used in the different metal processing industries are tested individually for their cooling capacity. From the content analysis, there are six kinds of charged ions found having a dominant amount in every type, namely Mg2+, SO42-, Ca2+, CO32-, Na+, and Cl-. Then, by using deionized water as a reference, the influence of the individual ion over the increase of Leidenfrost temperature is investigated. The results are then compared to the single-salt solution of MgSO4. Mg2+ and SO(4)(2-& nbsp;)are acknowledged as the most dominant ions contributing to the highest increase of Leidenfrost temperature. Comparing the influence of the salt mixture in the actual water to the single-salt solution will give insight not only on the most dominant specie but also its behavior when present as a mixture.
查看更多>>摘要：The author has proposed a model of brief interaction of two amorphous bodies with boundary conditions of the third and fourth kind in the amorphous body without a convective massflux. Consideration has been given to the thermophysical aspect of formation of the temperature sensation of a biomedical object (BMO) on contact with the amorphous body with a different temperature. The procedure of short measurements in the stage of irregular thermal regime (pulse method) was applied to investigation of thermal conductivity of various anatomical sections of the BMO skin as a function of its temperature. It has been shown experimentally that thermal conductivity grows with temperature and is within 0.29-0.48 W/(m K). The total relative error of the method of thermal conductivity measurements amounted to ~5%. It has been proposed that analytical calculation of the surface temperature of the BMO on contact be modeled as a combination of the temperatures in liquid and solid media by using the criterion of moisture-content percentage in the BMO.
查看更多>>摘要：Adsorption technology can supply both cooling and drinking water without using electricity as it can be operated using solar energy or waste heat sources. Commercially available adsorption systems utilize adsorbent porous materials such as zeolite and silica gel that suffer from high regeneration temperature for the former and lower hydrophilicity of the latter. To avoid these problems, aluminium fumarate metal-organic framework (MOF) material is used in this study as a promising new adsorbent material with water uptake higher than that of silica gel and zeolite. The material was integrated as a coated layer instead of the conventional granular form. The performance of the developed coated wire wound finned tube heat exchanger was compared with packed one using aluminium fumarate in adsorption cooling and desalination applications. Results showed that the performance of the coated heat exchanger outperformed the packed heat exchanger for both applications. For adsorption water desalination, the coated wire finned heat exchanger produced a specific daily water production (SDWP) of 23.5 m(3)/ton/day compared to only 12.7 m(3)/ton/day for packed heat exchanger. For adsorption cooling application, the coated wire finned heat exchanger produced a specific cooling power (SCP) and a coefficient of performance (COP) of 682 W/kg and 0.32 compared to 318.5 W/kg and 0.23 for packed heat exchanger.
查看更多>>摘要：The arch-fired boiler easily suffers from membrane wall deformation and rupture caused by high stress. Thus, it is necessary to predict thermal stress distribution on the membrane wall for the sake of early warning. In this study, a lab-scale arch-fired boiler was constructed to achieve thermal stress distribution on the membrane wall, and the reason for high-stress formation was found. Besides, machine learning technology was first applied to predict thermal stress based on experimental data. The results show that both high-temperature distribution and boiler configuration play an important role on high-stress distribution. As for the prediction, the regression neural network model presents an admissible result (R-2 = 0.724), but has a relatively poor performance on the high-stress prediction due to the skewed data distribution. Specifically, the relatively high prediction errors present on the bending and connection areas. Future work should consider the effect of boiler configuration and different heat loads. In contrast, the binary classification neural network model has a more accurate result with a high F1 score (0.893). It is the best choice for practical application considering the generality and accuracy. This work is meant to be valuable in optimizing the prediction model and applying it in practice.
查看更多>>摘要：Icing on wind turbine blades will affect the aerodynamic performance and damage the blade mass balance, which seriously reduce the power generation efficiency. In this paper, the heat transfer characteristics and anti-/deicing performance of a high heat transfer graphene oxide (GO) doped anti-/deicing component were investigated. Based on the numerical simulation analysis, the external flow field and internal temperature field distribution of the GO doped wind turbine blade anti-/deicing component was carried out, and the heat transfer ability of the anti-/deicing component was verified by heat transfer similarity experiment conducted under low temperature. The maximum error of the proposed heat transfer simulation model is 9.08%. The researches results showed that the heat transfer performance of anti-/deicing component was improved after GO doping. When the mass fraction of GO doping in the heat transfer layer was 0.10 wt%, the energy consumption was reduced by 5.40%. The operating parameters of the GO doped anti-/deicing component were optimized by response surface methodology. It is considered that anti-/deicing components of wind turbine blades had better anti-/deicing effect when the speed of wind turbine blade relative to ambient air was 15 m/s, the mass fraction of GO in heat transfer layer was 0.06 wt%, the heat flux of heating layer was 3195.02 W/m(2), and the external environment temperature was -10 degrees C.
NSTL
Elsevier