Passive solar heating of a non-airconditioned building with movable insulation over the roof pond

This paper presents a simple mathematical model for solar space heating in a non-airconditioned building with movable insulation over the roof pond. The building room considered is of rectangular shape (6 m × 5 m × 4 m) based on the ground. The effects of heat conduction to the ground, heat transfers to furnishings and heat losses due to air ventilation/infiltration have been taken into account in the general heat transfer analysis. The day-to-night change of insulation over the roof pond has been represented by a rectangular step function variation of the heat transfer coefficient at the pond's surface. An increase of 3 to 4°C in the room air temperature is achieved by means of movable insulation over the roof pond on a mild winter's day (17th February, 1982) in New Delhi.     

Analytical model of a solarium for cold climate—A new approach

In this present communication, a new approach has been evolved for analysing the thermal performance of a solarium useful in a cold climate. So as to trap the maximum solar radiation, the concept of glass windows (with movable insulation) in both the east and west walls of the sun space, besides in its south facing will and with the roof made of glass, has been introduced. Considering the ground temperature constant over a day, an overall heat transfer coefficient has been incorporated for the estimation of the heat flux transferred from the floor of the solarium to the ground. The introduction of this heat transfer coefficient eliminates the need of solving the conductivity equation for the heat flux conducted from the floor of the solarium to the ground. On account of the almost insulating behaviour of wood, an overall heat transfer coefficient has been taken for an all wooden structure of the solarium. Carrying out a transient analysis, explicit expressions for the temperatures of the sun space, the blackened absorbing surface of the water wall, the water itself, the living space and the isothermal masses have been developed as a function of time. These temperatures are required for the evaluation of the thermal energy taken in by the water wall, the heat flux entering the sun space and living space and the thermal energy distributed over the isothermal masses lying in the living space.     

Thermal performance of a non-air-conditioned building with PCCM thermal storage wall

This paper presents a time-dependent periodic heat transfer analysis of a non-air-conditioned building having a south-facing wall of phase-changing component material (PCCM). A rectangular room (6 × 5 × 4 m) based on the ground is considered. The effects of heat transfer through walls and roof, heat conduction to the basement ground and furnishings, heat gain through window and heat loss due to air ventilation have been incorporated in the periodic time-dependent heat transfer analysis. The time-dependent heat flux through the PCCM south-facing wall has been obtained by defining the effective thermal properties of the PCCM for a conduction process with no phase change. Numerical calculations are made for a typical mild winter day (7 March 1979) at New Delhi for heat flux entering through the wall and inside air temperature. Further, a PCCM wall of smaller thickness is more desirable, in comparison to an ordinary masonry concrete wall, for providing efficient thermal energy storage as well as excellent thermal comfort in buildings.     

Comparative numerical analysis of magnetized rectangular and trapezoidal fins

Fins are extended surfaces that are designed to dissipate heat from hot sources to their surroundings. The different profiles of fins are used on the equipment surface to improve heat transfer. Fins are extensively used in refrigeration, solar panels, superheaters, electric equipment, automobile parts, combustion engines, and electrical equipment. On the basis of these applications, we study the thermal performances of magnetized convective–radiative-rectangular fins with magnetized trapezoidal fins with internal heat generation. The shooting technique is used to numerically study the suggested model. It is revealed that magnetized trapezoidal fins transfer more heat than magnetized rectangular fins. It is also revealed that magnetized trapezoidal fins have higher thermal transfer competence than magnetized rectangular fins. When thermal conductivity, radiation–conduction number, and convection–conduction number increase, the fin's efficiency increases. In addition, a Hartmann number indicating the magnetic effect is found to improve heat transfer from the fins. Increasing the magnetism parameter from 0.1 to 0.3 reduced temperature by approximately 4.5%, changing internal heat generation from 0.1 to 0.5 increased temperature distribution by approximately 16%, and changing the Peclet number from 0.1 to 0.3 increased temperature distribution by approximately 15%. The effect of heat transfer coefficient, thermal radiation–conduction and convection–conduction, and dimensionless radiation are also investigated on the performance of the fins.     

Heat loss into ground from a slab‐on‐ground structure in a floor heating system

The objective of this research was to determine the actual heat loss into the subsoil from a massive slab‐on‐ground structure in a low temperature floor heating system. The main objective was achieved by field test measurements of an actual new building in Southern Finland. The test building is a detached house including a massive concrete slab, an underneath polystyrene insulation and a crushed stone fill layer on top of the clay subsoil. The heat loss into subsoil is determined from the measured temperature difference over the slab cross‐section during a 1‐year measuring period. The long‐term behaviour of the structure was also studied by numerical simulations using 2D FE‐modelling. According to the field test results and the simulations, the increase of the slab temperature in winter increases significantly the flow rates into the subsoil, also at the central part of the slab. Theoretical calculations for a standard building show that the heat loss into the subsoil from slab‐on‐ground structures is a significant part of the total heat loss from a building and the intensity of the heat loss is strongly dependent on the average temperature of the slab structure. The effect of the various floor heating systems on the total energy consumption of a building should be taken into consideration when designing the thermal insulation of ground slabs. Copyright © 2005 John Wiley & Sons, Ltd.     

The heat transfer characteristics of liquid cooling heatsink containing microchannels

This paper numerically and experimentally investigates the heat transfer performance and characteristics of liquid cooling heatsink containing microchannels. The effects of channel geometry and pressure drop between the entrance and exit of heatsink on the heat transfer performance are studied. The geometrical parameters include aspect ratio and cross-sectional porosity of the channels. The height of the microchannels is considered constant. The aspect ratio is set from 1.67 to 14.29 and the porosity is from 25% to 85%. The imposed pressure drop ranges between 490 and 2940 Pa. It is found that the aspect ratio corresponding to the lowest effective thermal resistance is changed with respect to the pressure drop. It is also noticed that the value of effective thermal resistance is almost a constant for cross-sectional porosity in the range of 53%–75%. The effective thermal resistance is increased when cross-sectional porosity is deviated from this range. In addition, the increasing of pressure drop enhances heat transfer performance for channels of high aspect ratio more than those of low aspect ratio.     

Bed-to-wall heat transfer modelling in the top region of a CFB riser column with abrupt riser exit geometries

The paper presents a mechanistic model to predict bed-to-wall heat transfer coefficient in the top region of a circulating fluidized bed (CFB) riser column by considering the riser exit geometry effects on bed hydrodynamics. With abrupt riser exit geometry, some solids will reflect back in to the riser column, thereby increasing the solids concentration in the top region of the riser column of a CFB. This in turn results in higher bed-to-wall heat transfer coefficients in the top region. At present, not much information exists in the literature to predict bed-to-wall heat transfer coefficient in the top region of a riser column with riser exit geometry effects. In the present work, a mechanistic model is proposed to estimate bed-to-wall heat transfer coefficient with riser exit geometry configurations. The length of influence of gas–solid flow structure from the riser exit due to various riser exit geometries is also presented. The solids reflux ratio is an important parameter, which influences the heat transfer rate in the top region. For the same operating conditions the bed-to-wall heat transfer coefficient increases with the abrupt riser exit geometry configuration compared to a smooth riser exit in the top region. The proposed model predictions are compared with the published experimental data for right angle exit configuration and a reasonable agreement is observed.     

Influence of internal channel geometry of gas turbine blade on flow structure and heat transfer

This paper presents the study of the influence of channel geometry on the flow structure and heat transfer,and also their correlations on all the walls of a radial cooling passage model of a gas turbine blade.The investigations focus on the heat transfer and aerodynamic measurements in the channel,which is an accurate representation of the configuration used in aeroengines.Correlations for the heat transfer coefficient and the pressure drop used in the design of internal cooling passages are often developed from simplified models.It is important to note that real engine passages do not have perfect rectangular cross sections,but include a comer fillets,ribs with fillet radii and a special orientation.Therefore,this work provides detailed fluid flow and heat transfer data for a model of radial cooling geometry which has very realistic features.     

扰流柱对间断交叉肋流动换热特性的影响研究

为了探究扰流柱对间断交叉肋通道流动与换热特性的影响,针对不同扰流柱数量和排布位置建立了不同的交叉肋模型,并通过数值模拟的方法,计算了各模型的阻力系数比、强化换热系数以及综合热效率3个性能指标的变化情况。研究结果表明：随着扰流柱数量的增大,阻力系数比和强化换热系数逐渐增大,而综合热效率不断下降;在进口雷诺数为20 000时,14柱模型与32柱模型相比,阻力系数比升高了15.4%,强化换热系数升高了32%,综合热效率提高了2.6%;将相同数量的扰流柱排布在通道内的不同位置对综合热效率的影响并不明显。     

Numerical Simulation of Natural Convection in a Porous Cavity with Linearly Temperature Distributions Under the Local Thermal Nonequilibrium Condition

A numerical investigation of the natural convection heat transfer in a rectangular cavity filled with a heat-generating porous medium by adopting the local thermal nonequilibrium (LTNE) model is reported in this paper. The top and bottom walls of the enclosure are adiabatic, the left wall is linearly cooled, and the right wall is cooled by a linear or uniform temperature profile. The results show that the isotherms for the fluid and solid phases become similar with the increase of the interphase heat transfer coefficient H; the increasing heat transfer between the two phases brings their temperatures closer to each other and thus the solid and fluid phases are in a state of the thermal equilibrium at higher values of H. For case A, the interphase heat transfer coefficient has little influence on the heat transfer rate of the solid phase of the porous cavity and the heat transfer profile of the solid phase (Nu_sy) is symmetrical with respect to the center point of line Y = 0.5. For case B, the interphase heat transfer coefficient H has a significant effect on that at the right wall, and the total heat transfer of the heat-generating porous cavity is implemented by the right-side wall. The total heat transfer rate Q of case B is higher than that of case A at a high thermal conductivity ratio γ (γ = 1, 10).     

Numerical investigation of turbulent heat transfer in channels with detached rib‐arrays

A numerical investigation is conducted to analyze the flow‐field and heat transfer characteristics in a rectangular passage of width‐to‐height ratio of 6:1 with detached ribs on one wall, where constant wall temperature condition is applied. The effect of detached‐rib geometry on heat transfer coefficient, friction factor, and thermal enhancement factor is investigated covering the range of the detached‐clearance ratios (c/a) of 0.1, 0.2, 0.3, and 0.4, the Reynolds number based on the channel hydraulic diameter ranges from 8000 to 24,000. The numerical results show that the flow‐field, temperature pattern, local Nusselt number distribution, average Nusselt number, and friction factor are strongly dependent on the detached‐clearance ratios. The thermal enhancement factor (TEF) under the same pumping power constraint is calculated in order to examine the overall effect of the detached‐clearance ratio. For the present range investigated, the maximum TEF of 1.22 is achieved by the use of the ribs with c/a of 0.1 at Reynolds number of 8000. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20357     

地埋管间断工作进出孔平均温度预测方法研究

按不改变每天总换热量的原则,将单位长度地埋管换热孔在制冷季或制热季中每天的实际释热或取热过程简化为一个矩形释热或取热脉冲,脉冲大小为单位长度地埋管换热孔的设计释热量或设计取热量,时间为每天的等效满负荷释热或取热小时数。采用线热源理论和热流叠加原理,推导若干个矩形脉冲负荷作用后地埋管换热器进出口温度平均值的计算公式,并通过长期现场岩土热响应试验对该公式进行了验证。在已知制冷季或制热季天数和地埋管换热器每天等效满负荷工作小时的基础上,通过设定地埋管在制冷季和制热季传热流体的最高或最低温度,可用该公式计算单位长度地埋管换热孔的设计释热量或取热量。     

Thermal enhancement of solar parabolic trough collectors by using nanofluids and converging-diverging absorber tube

Parabolic trough collectors are the most mature technology for utilizing the solar energy in high temperature applications. The objective of this study is the thermal efficiency enhancement of the commercial parabolic collector IST-PTC by increasing the convective heat transfer coefficient between the working fluid and the absorber. There are two main factors which influence on this parameter, the working fluid type and the absorber geometry. For this reason three working fluids are investigated, thermal oil, thermal oil with nanoparticles and pressurized water. Moreover, a dimpled absorber tube with sine geometry is tested because this shape increases the heat transfer surface and increases the turbulence in the flow. The final results show that these two techniques improve the heat transfer coefficient and the thermal efficiency of the collector. More specifically, the use of nanofluids increases the collector efficiency by 4.25% while the geometry improvement increases the efficiency by 4.55%. Furthermore, collector parameters such as the heat loss coefficient, the exergetic efficiency, the pressure losses and the absorber temperature are presented for all the examined cases. The model is designed with Solidworks and is simulated by its flow simulation studio.     

Optimum Thermal Analysis of a Heat Sink with Various Fin Cross-Sections by Adjusting Fin Length and Cross-Section

A theoretical analysis of a heat sink is presented in this study to pursue the purpose of maximum thermal dissipation and the least material cost. Due to the general derivation, the longitudinal fin arrays on a heat sink can have either square, rectangular, equilaterally triangular, or cylindrical cross-section. By input of the Biot number, Bi, heat transfer coefficient ratio, H, and the shape parameter, γ, the heat transfer equation, which is expressed in implicit form, can be solved by iterative method to calculate the optimum fin length and fin thickness. Meanwhile, the thermal resistance of a heat sink can be obtained to illustrate the cooling performance under various design conditions.     

Predicting the fluid temperature at the exit of the vertical ground heat exchangers

The energy analysis of ground source heat pump systems is based on the instantaneous fluid temperature at the ground heat exchanger outlet. This temperature defines the ground source heat pump coefficient of performance (COP) and hence the electricity consumption required in order to fulfill the energy demands of the building. The aim of this work is to present a model able to predict the fluid temperature at the ground heat exchanger outlet, taking into account the heat transfer phenomena in the soil and the temporal variation of the thermal load of the ground heat exchanger. The model developed was verified using experimental data, expanding over a three years period, of a vertical ground heat exchanger. It is proved that the model is able to satisfactorily predict the recorded temperature values throughout the verification period. The differences between measured and estimated outlet water temperatures impose a deviation between the estimated and the actually recorded electricity consumption of less than 4%.     

Effect of Side Ratio and Aiding/Opposing Buoyancy on the Aerodynamic and Heat Transfer Characteristics around a Rectangular Cylinder at Low Reynolds Numbers

A numerical investigation is carried out to understand the effect of side ratio, as well as aiding/opposing buoyancy on the aerodynamic and heat transfer characteristics around a rectangular cylinder in the vertical unconfined configuration. The representative vortex structures and isotherms patterns are presented and discussed. A correlation between the critical Richardson number for the buoyancy-induced breakdown of Kármán vortex street and the side ratio of the cylinder is obtained. The influence of side ratio and buoyancy on Strouhal number, drag and lift coefficient, the recirculation length, and heat transfer from the cylinder is also investigated.     

Conduction and entrance effects on laminar liquid flow and heat transfer in rectangular microchannels

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel spacings, namely 1 mm and 0.3 mm (0.1 mm), were used for three-dimensional (two-dimensional) numerical model, respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, thermal entrance effects and conduction/convection coupling effects are considered both for the test case of uniform channel inlet conditions and the complete geometry of the experiment. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect on heat transfer in microchannel heat sink down to the smallest size considered (0.1 mm).     

FLUID FLOW AND HEAT TRANSFER IN MULTIPLY-FOLDED,CONTINUOUS FLOW PASSAGES INCLUDING CONJUGATE THERMAL INTERACTION BETWEEN THE FLUID AND BOUNDING WALLS

A numerical simulation has been performed for the fluid flow and heat transfer in a flow passage consisting of a series of 180° bends connected by straight sections. The basic passage cross-section geometry is that of a flat rectangular duct. The simulation problem encompassed three-dimensional flows of both fluid and heat and involved the conjugate interaction of thermal phenomena in the fluid and in its bounding wall. The implementation of the numerical model was performed using FLUENT 5.0. Several results of practical interest were deduced from the simulation. With regard to the motivating application, the heating of biofluids prior to their introduction into the human body, local hotspots were found to occur whose presence could be injurious to living cells. The heating of the flowing fluid (a liquid to properly model the biofluid application) was found to be highly nonuniform at low values of the volumetric flowrate of the fluid despite the fact that the bounding walls of the flow passage were uniformly heated. Local-averaged heat transfer coefficients were evaluated at numerous locations along the length of the flow passage. Except for the regions of the bends of the passage, the local-averaged heat transfer coefficient was nearly constant and closely coincided with the fully developed heat transfer coefficient for flow in a flat rectangular duct.     

Thermal analysis of natural convection and radiation in porous fins

In this study, the effects of radiation and convection heat transfer in porous media are considered. The geometry considered is that of a rectangular profile fin. The porous fin allows the flow to infiltrate through it and solid-fluid interaction takes place. This study is performed using Darcy's model to formulate heat transfer equation. To study the thermal performance, three types of cases are considered viz. long fin, finite length fin with insulated tip and finite length fin with tip exposed. The theory section addresses the derived governing equation. The effects of the porosity parameter S_h, radiation parameter G and temperature ratio C_T on the dimensionless temperature distribution and heat transfer rate are discussed. The results suggest that the radiation transfers more heat than a similar model without radiation.