On the non-periodic unsteady heat transfer through walls

Existing calculation methods of the transient heat flow through walls for air-conditioning applications, assume periodic outdoor conditions. Therefore, the effect of a temporary temperature rise on the indoor heat flow is usually neglected. In the present work an attempt is being made to model the related non-periodic transient problem. The method of approach is based on a finite-difference solution of the transient heat conduction equation within the wall. Outdoor air temperature deviation parameters are introduced, which characterize any temporary deviation of the outdoor air-temperature from periodicity. Indoor heat flow deviation characteristics are defined, which describe the deviation of the indoor heat flow from periodicity, provoked by the corresponding outdoor air temperature deviation. A parametric study is conducted, where the effects of the temperature deviation parameters on the indoor heat flow deviation characteristics are examined. It has been found that (a) the maximum heat flow deviation varies linearly with the mean temperature difference, (b) the restoration ratio is practically independent of the temperature amplitude difference, and (c) an increase in the wall thermal diffusivity results in a decrease of the maximum heat flow deviation. The practical importance of the present analysis is that it helps towards the estimation of peak loads under non-periodic outdoor conditions.     

Nonlinear coupling between thermal mass and natural ventilation in buildings

An ideal naturally ventilated building model that allows a theoretical study of the effect of thermal mass associating with the non-linear coupling between the airflow rate and the indoor air temperature is proposed. When the ventilation rate is constant, both the phase shift and fluctuation of the indoor temperature are determined by the time constant of the system and the dimensionless convective heat transfer number. When the ventilation rate is a function of indoor and outdoor air temperature difference, the thermal mass number and the convective heat transfer air change parameter are suggested. The new thermal mass number measures the capacity of heat storage, rather than the amount of thermal mass. The analyses and numerical results show that the non-linearity of the system does neither change the periodic behaviour of the system, nor the behaviour of phase shift of the indoor air temperature when a periodic outdoor air temperature profile is considered. The maximum indoor air temperature phase shift induced by the direct outdoor air supply without control is 6 h.     

Effects of Non-Darcian and Inlet Conditions on the Forced Convection Along a Vertical Plate With Film Evaporation

The present study analyzes theoretically the non-Darcian effects and inlet conditions of forced convection flow with liquid film evaporation in a porous medium. The physical scheme includes a liquid–air streams combined system; the liquid film falls down along the plate and is exposed to a cocurrent forced moist air stream. The axial momentum, energy, and concentration equations for the air and water flows are developed based on the steady two-dimensional (2-D) laminar boundary layer model. The non-Darcian convective, boundary, and inertia effects are considered to describe the momentum characteristics of a porous medium. The paper clearly describes the temperature and mass concentration variations at the liquid–air interface and provides the heat and mass transfer distributions along the heated plate. Then, the paper further evaluates the non-Darcian effects and inlet conditions on the heat transfer and evaporating rate of liquid film evaporation. The numerical results show that latent heat transfer plays the dominant heat transfer role. Carrying out a parametric analysis indicates that higher air Reynolds number, higher wetted wall temperature, and lower moist air relative humidity will produce a better evaporating rate and heat transfer rate. In addition, a non-Darcy model should be adopted in the present study. The maximum error for predictions of heat and mass transfer performance will be 21% when the Darcy model is used.     

Mixed convection heat transfer enhancement through film evaporation in inclined square ducts

The investigation of mixed convection heat transfer enhancement through film evaporation in inclined square ducts has been numerically examined in detail. The main parameters discussed in this work include the inclined angle, the wetted wall temperature and the relative humidity of the moist air mixture. The numerical results of the local friction factor, Nusselt number and Sherwood number are presented for moist air mixture system. Attention was particular paid to the effects of latent heat transport on the heat transfer enhancement. Results show that the latent heat transport with film evaporation augments tremendously the heat transfer rate. The heat transfer rate can be enhanced to be 10 times of that without mass transfer, especially for a system with a lower temperature. Besides, better heat and mass transfer rates related with film evaporation are found for case with a higher wetted wall temperature. The increase in the relative humidity of moist air in the ambient causes the decrease in heat transfer enhancement.     

Numerical analysis of a PCM thermal storage system with varying wall temperature

Numerical analysis of melting and freezing of a PCM thermal storage unit (TSU) with varying wall temperature is presented. The TSU under analysis consists of several layers of thin slabs of a PCM subjected to convective boundary conditions where air flows between the slabs. The model employed takes into account the variations in wall temperature along the direction of air flow as well as the sensible heat. The paper discusses typical characteristics of the melting/freezing of PCM slabs in an air stream and presents some results of the numerical simulation in terms of air outlet temperatures and heat transfer rates during the whole periods of melting and freezing. Considerations in the design of the TSU are also given.     

Passive heat and moisture removal from a natural vented enclosure with a massive wall

Simultaneous transport of heat and moisture by conjugate natural convection in a partial enclosure with a solid wall is investigated numerically. Moist air motions are driven by the external temperature and concentration differences imposed across enclosures with different ambient moisture conditions. The Prandtl number and Schmidt number used are 0.7 and 0.6, respectively. The fluid, heat and moisture transports through the cavity and solid wall are, respectively, analyzed using the streamlines, heatlines and masslines, and the heat and mass transfer potentials are also explained by the variations of overall Nusselt and Sherwood numbers. The numerical simulations presented here span a wide range of the main parameters (heat and mass diffusion coefficient ratios, solid wall thickness and thermal Rayleigh numbers) in the domain of aiding and opposing buoyancy-driven flows. It is shown that the heat transfer potential, mass transfer potential, and volume flow rate can be promoted or inhibited, depending strongly on the wall materials and size, thermal and moisture Rayleigh numbers.     

Laminar mixed convection heat and mass transfer in vertical rectangular ducts with film evaporation and condensation

The investigation of mixed convection heat and mass transfer in vertical ducts with film evaporation and condensation has been numerically examined in detail. This work is primarily focused on the effect of film evaporation and condensation along the wetted wall with constant temperature and concentration on the heat and mass transfer in rectangular vertical ducts. The numerical results, including the distributions of dimensionless axial velocity, temperature and concentration distributions, Nusselt number as well as Sherwood number are presented for moist air mixture system with different wall temperatures and aspect ratios of the rectangular ducts. The results show that the latent heat transport with film evaporation and condensation augments tremendously the heat transfer rate. Better heat transfer enhancement related with film evaporation is found for a system with a higher wall temperature.     

Dynamic defrosting characteristics of air source heat pump and effects of outdoor air parameters on defrost cycle performance

The reverse-cycle defrosting characteristics of a 11.2 kW split-type air source heat pump (ASHP) were experimentally investigated under the defrosting conditions. Based on the experimental results, the effects of outdoor air parameters on defrosting cycle performance as well as the dynamic defrosting characteristics of the ASHP unit were analyzed. The experimental results indicate that with the increase of the outdoor air relative humidity at a constant air temperature and velocity, the total power consumption, defrosting time and endotherm from indoor room during defrosting period decreased linearly, and they also decreased as the outdoor air temperature increased, but the trends of the curves presented the concave-up. The duration of the defrosting period mainly depends on the wall temperature of outdoor heat exchanger, which is corresponding to the condensing pressure during the defrosting cycle. In this paper, the concept of total coefficient of performance (COP) is used to evaluate the performance of ASHP unit, and as the air temperature increased under the conditions of a fixed air relative humidity and air velocity, the total COP increased linearly, but it decreased linearly as the air relative humidity increased.     

Combined heat and mass transfer in natural convection on a vertical flat plate

In analyzing the combined heat and mass transfer in natural convection, most of the surface conditions are either maintained at an uniform wall temperature and uniform wall concentration or subjected to an uniform heat flux and uniform mass fluc. Other conditions are seldom investigated. This study is to investigate the effects of the coupled thermal and mass diffusion on the natural convection of a vertical plate for a moist air system. The surface conditions of the plate are uniform heat flux and uniform relative humidity. A finite difference numerical method is used to solve the governing equations simultaneously. The results that the relative humidity of the surface is both larger and smaller than that of the ambient are examined in detail.     

Experimental and numerical validation of hygrothermal transfer in brick wall

In this paper, a mathematical model dealing with a coupled heat, air, and moisture transfer in a building envelope was developed. Based on the three-following driving potential: vapor pressure, dry air pressure, and temperature, an application on a hygrothermal behavior of a real wall was carried out for different climatic conditions. For this purpose, a characterization of the heat and moisture properties of the materials constituting the wall made with red brick and cement mortar was carried out in the laboratory. This was used to evaluate experimentally the input parameters of the model as a function of relative humidity. To validate the numerical model, an experimental platform was improved. The wall was set up in a double-climatic chamber with different boundary conditions, and then the temperature and humidity evolutions were recorded using several sensors within the wall thickness. The results have highlighted a good agreement between numerical simulation results and experimental ones.     

An indirect evaporative chiller

A novel indirect evaporative chiller driven by outdoor dry air to produce cold water as the cooling source for air conditioning systems is introduced, and the principle and the structure of the chiller is presented. The cold water can be produced almost reversibly under ideal working conditions, with its temperature infinitely close to the dew point temperature of the inlet air. The key components of the chiller are an air cooler and a padding tower. To improve the heat transfer performance inside the chiller, a quasi-countercurrent air cooler was designed; a subsection linear method was used for the mathematical model of the padding tower. The first indirect evaporative chiller, designed and developed in 2005, has been in use in Kairui Building, a big hotel in Shihezi, Xinjiang Autonomous Region. The tested temperature of the water produced is below the wet bulb temperature of outdoor air and reached the average value of the dew point temperature and the wet bulb temperature of outdoor air. As the running components are only pumps and fans, the COP (cooling energy for room divided by power cost) of this chiller is high, and the drier the outdoor air, the higher COP the chiller obtained. Since no CFCs are used in this chiller, it would not cause any pollution to the aerosphere. Finally, the application prospect of the indirect evaporative chiller in the world is presented.     

Assessment of Building External Wall Thermal Performance Based on Temperature Deviation Impact Factor under Discontinuous Radiant Heating

As the variation and timely meeting thermal environment requirement of indoor air temperature has a close relationship with the thermal performance of building external wall under discontinuous radiant heating condition, one appropriate assessment method or index for assessing the building external wall thermal performance is very necessary. In order to reasonably evaluate the thermal performance of external wall under discontinuous radiant heating condition and build the direct connections and interactions among the indoor air temperature, external wall inner surface temperature and outdoor air temperature, the first and second impact factors of temperature deviation were established, based on one mathematical model of room heat transfer. For one experimental room and four types of external walls under discontinuous radiant heating condition, both the influence of the external wall inner surface temperature deviation on the indoor air temperature and that of the outdoor air temperature deviation on the external wall inner surface temperature were determined effectively with the first and second impact factors of temperature deviation. In addition, favourable performance for the self-insulation and inner insulation walls were found, due to their superiority in effectively and timely improving the indoor thermal environment under discontinuous radiant heating condition.     

Transient effects in natural convection cooling of vertical parallel plates

This work presents results from a numerical study of transient natural convection between vertical parallel plates. Two boundary conditions – uniform wall temperature and uniform heat flux – are considered. Results presented include the rate of heat transfer for uniform wall temperature and the maximum wall temperature for uniform heat flux. Also presented are simple correlations to calculate the minimum heat transfer and the maximum wall temperature during the transient period. It is found that for uniform wall temperature the ratio of the minimum heat transfer to the steady state heat transfer decreases with length of the channel, and for uniform heat flux the maximum transient temperature has a maximum of about 9% over the steady state temperature.     

Hybrid formulation and solution for transient conjugated conduction–external convection

This work presents a hybrid numerical–analytical solution for transient laminar forced convection over flat plates of non-negligible thickness, subjected to arbitrary time variations of applied wall heat flux at the fluid–solid interface. This conjugated conduction–convection problem is first reformulated through the employment of the coupled integral equations approach (CIEA) to simplify the heat conduction problem on the plate by averaging the related energy equation in the transversal direction. As a result, an improved lumped partial differential formulation for the transversally averaged wall temperature is obtained, while a third kind boundary condition is achieved for the fluid from the heat balance at the solid–fluid interface. From the available steady velocity distributions, a hybrid numerical–analytical solution based on the generalized integral transform technique (GITT), under its partial transformation mode, is then proposed, combined with the method of lines implemented in the Mathematica 5.2 routine NDSolve. The interface heat flux partitions and heat transfer coefficients are readily determined from the wall temperature distributions, as well as the temperature values at any desired point within the fluid. A few test cases for different materials and wall thicknesses are defined to allow for a physical interpretation of the wall participation effect in contrast with the simplified model without conjugation.     

Transient model for coupled heat,air and moisture transfer through multilayered porous media

Most building materials are porous, composed of solid matrix and pores. The time varying indoor and outdoor climatic conditions result heat, air and moisture (HAM) transfer across building enclosures. In this paper, a transient model that solves the coupled heat, air and moisture transfer through multilayered porous media is developed and benchmarked using internationally published analytical, numerical and experimental test cases. The good agreements obtained with the respective test cases suggest that the model can be used to assess the hygrothermal performance of building envelope components as well as to simulate the dynamic moisture absorption and release of moisture buffering materials.     

A combined experimental and numerical study of thermal processes, performance and nitric oxide emissions in a hydrogen-fueled spark-ignition engine

This work concerns the study of a spark-ignition engine fueled with hydrogen, using both measured and numerical data at various conditions, focusing on the combustion efficiency, the heat transfer phenomena and heat loss to the cylinder walls, the performance, as well as the nitric oxide (NO) emissions formed, when the fuel/air and compression ratio are varied. For the investigation of the heat transfer mechanism, the local wall temperatures and heat flux rates were measured at three locations of the cylinder liner in a CFR engine. These fluxes can provide a reliable estimation of the total heat loss through the cylinder walls and of the hydrogen flame arrival at specific locations. Together with the experimental analysis, the numerical results obtained from a validated in-house CFD code were utilized for gaining a more complete view of the heat transfer mechanism and the hydrogen combustion efficiency for the various cases examined. The performance of the CFR engine is then identified, since the calculated cylinder pressures are compared with the measured ones, from which performance and heat release rates are calculated and discussed. Further, NO emission studies have been accomplished, with the calculated results not only being compared with the measured exhaust NO ones, but also further processed for conducting an in-depth investigation of the dependence of NO production on the spatial distribution of in-cylinder gas temperature. It is revealed that for lower fuel/air ratio the burned gas temperature is held at low level and the heat loss ratio is quite low. As the load increases and stoichiometric mixtures are used, the wall and in-cylinder gas temperatures increase substantially, together with the heat loss and the NO emissions, owing to the high hydrogen combustion velocity and the consequent high rate of temperature rise. The combustion efficiency is slightly increased, but the indicated efficiency is decreased due to higher heat loss.     

腔式太阳能高温空气吸热器传热过程数值模拟

介绍了一种应用于塔式太阳能热发电站的腔式高温空气吸热器,建立了吸热器内部空气流动及传热过程模拟数学模型,并通过数值方法,模拟了吸热器内部的空气流场和温度场。结果得知:空气进入吸热器后,沿内壁面轴向高速流动,随着深度的增加,速度越来越小,到达底部时速度最小;在压差的作用下,进入吸热器内部的空气会不断流向和冲刷针肋及壁面,而主流方向的流量不断减少;空气通过冲刷高温针肋及壁面不断吸收热量,温度不断升高;由于吸热器底部空气速度较小,对流换热系数较小和热流密度较大,因此该处温度较高,是整个吸热器的最脆弱部位;在高辐照强度情况下,虽然加大空气流量可降低吸热器壁面的温度,但由于其对流换热系数与空气流速不成正比例,壁面温度一般还会有所升高。     

Numerical simulation and comparison with experiment of natural convection between two floors of a building model via a stairwell

The paper presents a numerical study of three-dimensional buoyancy-driven flow in a half-scale model of a two-floor building model. The model consists of an upper compartment and a lower compartment with a stairway connecting the two floors. The model forms a closed system, with no inlet or outlet. The flow is driven by a single heat source placed in the lower compartment. The study is linked closely to a previously published experimental study by the present author, which provided the details of the geometry and the boundary conditions as well as data for comparison with the present numerical results. The numerical method is large eddy simulation with the dynamic kinetic energy transport subgrid model. Radiation exchange is modelled using the discrete ordinates (DO) radiation model. The thermal boundary conditions on the model walls are set as heat flux. It is shown that the air temperature level is sensitive to the initial conditions for temperature, but air velocity is unaffected. In order to study this effect further, with the aid of the k–ε model, the measured wall temperatures are set as boundary conditions, which removes the dependency on initial temperature. For the cases studied, comparisons are made between the measured and computed wall temperatures, wall heat fluxes, air temperature and air velocity. There is a general agreement between the two results.     

几种带冷却结构的双层管换热及流动对比

对相同质量流量下的光管、双层光管、带冷却结构（肋、扰流柱、凹坑、螺旋通道）的双层管等不同结构的管流动进行了流固耦合三维数值模拟,获取了固体壁温的分布特征;对各结构下,外层壁冷热侧温差、冷气温升、流动特性及综合换热效率进行了研究分析。研究结果表明：相同质量流量下,带螺旋通道双层管的外层壁冷热侧温差最小、综合换热效率最高;凹坑结构双层管与双层光管的流动及换热特性相似,流阻较小但换热效果也较差;扰流柱和肋结构双层管的流动换热特性相近,其温度分布均匀性、换热量介于双层光管和螺旋通道双层管之间,其流阻大且综合换热效率低。     

Performance improvement of PV/T solar collectors with natural air flow operation

The electrical efficiency of a photovoltaic system drops as its operating temperature rises and PV cooling is necessary. The photovoltaic/thermal (PV/T) system is a relatively recent type of solar collector where a circulating fluid of lower temperature than PV module extracts heat from it, cooling the module to improve its output power while the solar pre-heated fluid provides sensible heat. In the present work, air cooling of a commercial PV module configured as PV/T air solar collector by natural flow is presented, where two low cost modification techniques to enhance heat transfer to air stream in the air channel are studied. The considered methods consist of thin metal sheet suspended at the middle or fins attached to the back wall of the air-channel to improve heat extraction from the module. A numerical model was developed and validated against the experimental data obtained from outdoor test campaigns for both glazed and unglazed PV/T prototype models studied. The validation results show good agreement between predicted values and measured data and thus could be used to study analytically the performance of these PV/T air collectors with respect to several design and operating parameters. The modified systems present better performance than the usual type and will contribute to better performance of integrated PV systems for natural ventilation applications in buildings, both space cooling and heating.