An active charge, passive discharge floor space heating system

This space heating system has a rockbed beneath and in contact with the floor of a dwelling, which is heated by radiation and convection from the floor. The ability of the heating system to maintain comfort conditions with no additional energy input is discussed and it is shown that the system is more suitable for use in mild climates than severe ones. Experimental work on horizontal air flow rockbeds is reported and shows that shallow beds can be designed in the same way as vertical air flow beds. The influence of natural convection on the effective thermal conductivity of the experimental rockbeds is reported.     

渐变孔隙率对泡沫铝矩形通道热沉流动和换热性能的影响

风冷是最普遍的电子器件散热方式。对填充渐变孔隙率泡沫铝的热沉的流动与换热性能进行CFD模拟,考察了孔隙率分别沿流动方向和高度方向发生梯度变化对热沉性能的影响。结果表明:与均匀孔隙率热沉相比,孔隙率沿高度方向渐变的热沉压力损失减小,最佳的孔隙率渐变方式为沿高度方向由0.963 0递减至0.700 0,与当量孔隙率0.831 5的均匀孔隙率热沉相比,明显提高了泡沫铝热沉的综合性能。当进口速度高于3.0 m/s后,渐变孔隙率热沉的综合性能甚至优于高孔隙率(0.963 0)的均匀孔隙率热沉。     

Numerical simulation of convective heat transfer between air flow and ceramic foams to optimise volumetric solar air receiver performances

Porous ceramic foams are used to achieve high performance in solar heat recovery systems. Understanding the convective heat transfer between the air flow and the ceramic foam is of great importance when optimising the volumetric air receiver. In this work, the convective heat transfer was numerically studied. The present approach was designed to compute the local convective heat transfer coefficient between the air flow and a porous ceramic foam. For that purpose, the energy balance and the flow inside the porous ceramic foam were solved. In addition, a detailed geometry of the porous ceramic foam was considered. The ceramic foams were represented by idealised packed tetrakaidecahedron structures. The numerical simulations were based on the three dimensional Reynolds-averaged Navier–Stokes (RANS) equations. A sensitivity study on the heat transfer coefficient was conducted with the porosity, velocity and mean cell size as parameters. Based on the numerical simulation results, a correlation for the volumetric local convective heat transfer coefficient between air and ceramic foams was developed. The resulting correlation covers a wide range of porosities, velocities, cell sizes and temperatures. The correlation results were compared with experimental data from the literature, and the comparison shows good agreement. The correlation is intended to be used in the design of volumetric solar air receivers.     

High efficiency solar air heater

This article presents an analysis for a novel type of solar air heater. The main idea is to minimize heat losses from the front cover of the collector and to maximize heat extraction from the absorber. This can be done by forcing air to flow over the front glass cover (preheat the air) before passing through the absorber. Hence, this design needs an extra cover to form a counter-flow heat exchanger. Porous media forms an extensive area for heat transfer, where the volumetric heat transfer coefficient is very high. Hence, using a porous absorber will enhance heat transfer from the absorber to the airstream. In the design of this type of collector, which combines double air passage and porous media, care should be taken to minimize pressure drop. However, the thermal efficiency of this type of collector is significantly higher than the thermal efficiency of conventional air heaters. The thermal efficiency of the suggested collector exceeds 75% under normal operating conditions. The pressure drop is not so significant if high porous medium is used and careful design of U-return section is considered.     

Frosting of heat pump with heat recovery facility

This paper aims to prolong the heat pump frost time and reduce its growth with heat recovery facility, which should mix the exhausted indoor and outdoor air before entering the evaporator. An ideal mathematic model is developed for heat transfer, frost generation and airside pressure drop. The properties of the mixture would be obtained by solving the mass and energy conservation equations. A parametric analysis is performed to investigate the effects of air inlet temperature, relative humidity and air mass flow rate on total heat transfer coefficient, frost thickness and airside pressure drop, respectively. The results show that rationalizing the ratio of indoor and outdoor air could prolong frosting time and reduce the frost thickness greatly. The total heat transfer coefficient, frost thickness and airside pressure drop increase monotonically with time going, but are not proportional. Decreasing the mixture inlet air temperature and relative humidity could essentially reduce frost growth on the tube surfaces. This can also be observed when increasing the air mass flow rate.     

Experimental investigation into the thermal-flow performance characteristics of an evaporative cooler

This study investigates the thermal-flow performance characteristics of an evaporative cooler. The derivation of the Poppe [1] and Merkel [2] analysis for evaporative coolers are presented and discussed. Performance tests were conducted on an evaporative cooler consisting of 15 tube rows with 38.1 mm outer diameter galvanized steel tubes arranged in a 76.2 mm triangular pattern. From the experimental results, correlations for the water film heat transfer coefficient, air–water mass transfer coefficient and air-side pressure drop are developed. The experimental tests show that the water film heat transfer coefficient is a function of the air mass velocity, deluge water mass velocity as well as the deluge water temperature, while the air–water mass transfer coefficient is a function of the air mass velocity and the deluge water mass velocity. It was found that the correlations obtained for the water film heat transfer coefficient and the air–water mass transfer coefficient compare well with the correlations given by Mizushina et al. [3]. Relatively little published information is available for predicting the air-side pressure drop across deluged tube bundles. The present study shows that the pressure drop across the bundle is a function of the air mass velocity and the deluge water mass velocity.     

Heat transfer and pressure drop experimental correlations for air–water bubbly flow

In this paper, a novel air–water bubbly flow heat transfer experiment is performed to investigate the characteristics of pressure drop of airflow and heat transfer between water and tubes for its potential application in evaporative cooling. The attempts to reduce the pressure drop while maintaining higher heat transfer coefficient have been achieved by decreasing the bubble layer thickness through the water pump circulation. Pressure drops of air passing through the sieve plate and the bubbling layer are measured for different height of bubble layer, hole–plate area ratio of the sieve plate and the superficial air velocity. Experimental data show that the increase of bubble layer height and air velocity both increase the pressure drop while the effect of the hole–plate area ratio of the sieve plate on the heat transfer coefficient is relatively sophisticated. A pressure drop correlation including the effects of all the tested parameters is proposed, which has a mean absolute deviation of 14.5% to that of the experimental data. Heat transfer coefficients of the water and the outside tube wall are measured and the effects of superficial air velocity, heat flux and bubble layer height are also examined. Through a dimensional analysis, a heat transfer correlation with a mean absolute deviation of 9.7% is obtained based on experimental data.     

泡沫铝通道内瞬态流动的平均换热系数

针对泡沫铝金属填充矩形通道内的对流换热开展了瞬态实验研究,分析了泡沫铝孔径(孔隙率)、流体流量(流速)等关键参数的影响。为了有效地处理实验数据,重新定义并推导了平均换热系数的计算公式,得到了泡沫铝通道内流动的平均换热系数,并引入了基于渗透率的雷诺数和达西数,确定了相关换热、流动准则数关系。实验研究表明,流速的增大有利于对流换热的强化:而平均换热系数对泡沫金属孔径较敏感;对于低孔隙率泡沫金属,渗透率成为影响换热强度的主要因素,相同或接近的孔隙率下,孔径越大,渗透率和达西数越大,越有利于换热,且压损减小。     

Performance enhancement of a split air conditioner using porous material inside the evaporator pipes

In this experimental study, a porous material is used inside the pipes of the evaporator as the main heat exchanging device in the air conditioning cycle. The used porous material consists of stainless steel balls of different diameters. As a case study, refrigerant R454B, which is a drop-in replacement to refrigerant R410A, is used as a working fluid in the air conditioner thermodynamic cycle. Four different porosities were used during the experimental tests; 100% (empty tube), 46%, 40%, and 33%. This study investigated the influence of variation of porosity as well as outside air temperature and refrigerant evaporation temperature on the cycle coefficient of performance, evaporation capacity, pressure drop, and power consumption during the compression process. Measured evaporation temperatures and indoor temperatures during tests were in the range of 1.5–12°C and 18–25°C, respectively. The use of porous material in the evaporation heat exchanger resulted in a considerable increase in the cycle evaporation capacity and coefficient of performance. Varying porosity from 100% to 33% resulted in an average percent increase of cycle evaporation capacity and coefficient of performance by 48.8% and 84.3%, respectively. Also, decreasing porosity from 100% to 33% resulted in an average percent increase in power consumption during the compression process by about 27%. An average percent increase of power consumption of compressor by about 25.9% is also reported, when evaporation temperature increased from 1.5°C to 12°C. Increasing outside air temperature from 27.1°C to 39.5°C resulted in decreasing evaporation capacity and coefficient of performance by 35.2% and 34.5%, respectively, and in increasing compressor power consumption by about 14.3%. A considerable pressure drop was recorded during the evaporation process when using porous material. The volumetric evaporation capacity, as well as compressor discharge temperature, are increased by increasing evaporating temperature and by decreasing evaporator porosity. The increase in air ambient temperature resulted in a considerable increase in refrigerant mass flow rate.     

Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

低气压环境下开缝翅片管换热器换热性能研究

利用低气压环境模拟装置对开缝翅片管换热器在不同气压下的换热性能进行实验研究.研究结果表明:随着气压不断降低,换热器周围空气密度逐渐降低,换热器空气侧换热系数以及显热换热量逐渐降低,而空气含湿量随着气压降低逐渐升高,导致潜热换热量逐渐增加;当气压降至0.058 MPa以下时,换热器空气侧潜热换热量占主要部分,当气压为0.04 MPa时,换热器换热能力与常压下相比下降了36.63%.     

Thermofluid characteristics of frosted finned-tube heat exchangers

The performance of frosted finned-tube heat exchangers of different fin types is investigated by experiments in this paper. The effects of the air flow rate, the air relative humidity, the refrigerant temperature, and the fin type on the thermofluid characteristics of the heat exchangers are discussed. The time variations of the heat transfer rate, the overall heat transfer coefficient, and the pressure drop of the heat exchangers are presented. The heat transfer rate, the overall heat transfer coefficient, and the pressure drop for heat exchangers with re-direction louver fins are higher than those with flat plate fins and one-sided louver fins are. The amount of frost formation is the highest for heat exchangers with re-direction louver fins.     

Heat transfer and pressure drop characteristics of peripheral-finned tube heat exchangers

The peripheral-finned tube is a new geometry aimed at avoiding moisture-condensate blockage hindering of the air-side heat transfer, by allowing for robust air flow pathways. It consists of a porous structure formed by periodic, radial-hexagonal fin arrangements of different radial extents mounted with a 30° offset from its neighboring level. Here, the air-side pressure drop and the heat transfer characteristics of five different heat exchanger prototypes with different geometric characteristics, such as the radial fin length, fin distribution, and heat exchanger length, were evaluated experimentally in an open-loop wind-tunnel calorimeter. The results demonstrate the effective performance, i.e., the pressure drop and heat transfer characteristics, of this new heat exchanger. A one-dimensional theoretical model based on the porous media treatment was also developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny–Carman model and the particle-diameter based analysis. The model predicts the experimental data within a few percent RMS, depending on the correlations used for the friction coefficient and interstitial Nusselt number.     

Performance of Forced Convection Heat Transfer in Porous Media Based on Gibson–Ashby Constitutive Model

A novel simulation model is developed for predicting the performance of forced convection heat transfer in the porous metal foam. Based on the physical geometry of the Gibson-Ashby constitutive model, the theoretical model proposed is able to predict the mechanical behaviors and thermal physical properties of porous materials simultaneously. The theoretical predictions of the overall heat transfer coefficient and pressure drop were compared with available experimental data for two different porous foam tubes. The first tube has a porous diameter of 0.6mm and porosity of 0.402, and the other tube has a diameter of 1.6mm and porosity of 0.462. The results show that the relative deviation of the flow pressure drop between the prediction and the experimental data are in the range from 5% to10% while the relative deviation of the overall heat transfer coefficient is about 20%. These deviations are acceptable for applications in engineering. So the feasibility of the Gibson-Ashby constitutive model to be used to predict the performance of flow resistance and convective heat transfer in porous foam ducts is satisfactorily validated.     

Performance of a double pass solar air collector

Double pass counter flow solar air collector with porous material in the second air passage is one of the important and attractive design improvement that has been proposed to improve the thermal performance. This paper presents theoretical and experimental analysis of double pass solar air collector with and without porous material. A mathematical model has been developed based on volumetric heat transfer coefficient. Effects of various parameters on the thermal performance and pressure drop characteristics have been discussed. Comparison of results reveals that the thermal efficiency of double pass solar air collector with porous absorbing material is 20-25% and 30-35% higher than that of double pass solar air collector without porous absorbing material and single pass collector respectively.     

Heat transfer and friction factor correlations for packed bed solar air heater for a low porosity system

An experimental investigation has been carried out on a low porosity packed bed solar air heater. This investigation covers a wide range of geometrical parameters of wire screen matrix, i.e. wire diameter 0.795 to 1.40 mm, pitch 2.50 to 3.19 mm and number of layers from 5 to 12. The correlations have been developed for the Colburn j factor and friction factor for a low range of porosities from 0.667 to 0.880 and packing Reynolds number range from 182 to 1168. It is observed that both the heat transfer coefficient and the friction factor are strong functions of geometrical parameters of the porous packed bed. A decrease in porosity increases the volumetric heat transfer coefficient.     

Comparison of Heat Transfer and Pressure Drop for the Helically Baffled Heat Exchanger Combined with Three-Dimensional and Two-Dimensional Finned Tubes

Experiments were performed to compare the shell-side heat transfer coefficient and pressure drop of a helically baffled heat exchanger with petal-shaped finned tubes to those of low-finned tubes for oil cooling using water as a coolant. The experimental results showed that for the heat exchanger with petal-shaped finned tubes, the shell-side heat transfer coefficients were augmented by 28–48%, yet the shell-side pressure drops were reduced by 35–75% at the same volumetric flow rates of oil. The possible mechanisms responsible for this heat transfer enhancement were analyzed for helically baffled heat exchanger combined with petal-shaped finned tubes.     

Performance of finned tube heat exchangers operating under frosting conditions

In this paper, the performance of flat plate finned tube heat exchangers operating under frosting conditions was investigated experimentally. Heat exchangers of single and multiple tube row(s) were tested to show the effects of various parameters on heat transfer performance. The parameters include temperature and relative humidity of air, flow rate of air, refrigerant temperature, fin pitch, and row number. The time variations of heat transfer rate, overall heat transfer coefficient, and pressure drop of heat exchangers presented.     

微细管束换热器结霜及流动对流换热特性实验研究

利用可视化换热器性能实验台测试了微细管束表面在无换热、大温差、凝露条件下不同风速工况的流动损失与空气侧换热系数。同时探究了超低温工况下,相对湿度和风速对微细管束换热器流动换热特性影响。相对湿度增大,湿空气释放潜热增加,结霜量增大,换热器前后压损增加。高速气流具有剪切作用,风速增大会导致结霜迟滞。     

Thermal and hydraulic analysis of a brazed aluminum evaporator

This paper presents an analytical/computer model to predict the performance of a brazed aluminum evaporator operating under dehumidifying conditions. The evaporator uses small hydraulic diameter, flat multi-channel tubes and louver fins. The in-tube refrigerant flow was divided into three regions including the two-phase, liquid deficient and superheat regions. For each region, correlations were selected from the open literature to calculate the local heat transfer and pressure drop. The effects of refrigerant pressure drop along tube and pressure losses at the tube entrance and exit were accounted for in the heat transfer calculations. The air-side fins were assumed to operate at the fully wet condition and the sensible heat transfer coefficient of the wet fins was assumed to be equal to that of the dry fins. The overall heat transfer coefficient was calculated using the enthalpy driving potential method. The total heat transfer rate and refrigerant pressure drop depend on the ratio of the number of tubes in the first and second passes. Parametric studies were done to illustrate selection of the preferred number of tubes per pass. The average refrigerant side heat transfer coefficient is sensitive to the dry-out vapor quality. However, the total heat transfer rate is relatively insensitive to the dry-out vapor quality. As the air inlet humidity increases, the latent and total heat transfer rates increase, but the sensible heat transfer rate decreases. The program was used to design an R-404A evaporator, for which a prototype was built and tested. The program over-predicted the evaporator capacity by 8%. The over-prediction is believed due to flow mal-distribution in the branch tubes.