Short time step heat flow calculation of building constructions based on frequency-domain regression method

Short time step heat flow calculation of building constructions is often needed for practical applications. Conventional methods such as state-space method and root-finding method may produce unstable conduction transfer function (CTF) coefficients at short time steps, and thus result in unstable heat flow calculation through building constructions. Frequency-domain regression (FDR) method is a newly developed method for computing CTF coefficients efficiently by representing the real building construction system with equivalent polynomial s-transfer functions. Previous studies on this method mainly addressed CTF coefficients at the conventional time step of 3600 s and the performance of heat flow calculation using these coefficients. This paper presents the investigation on the performance of CTF coefficients at various short time steps based on FDR method, and the performance of the heat flow calculation using these coefficients. The results show that FDR method can produce stable CTF coefficients at various time steps for most building constructions, and the calculated heat flows using these coefficients are of high accuracy.     

Investigation of local heat transfer coefficients in plate heat exchangers with temperature oscillation IR thermography and CFD

A method for the measurement of local convective heat transfer coefficients from the outside of a heat-transferring wall has been developed. This method is contact-free and fluid independent, employing radiant heating by laser or halogen spotlights and an IR camera for surface temperature measurements; it allows for the rapid evaluation of the heat transfer coefficient distribution of sizable heat exchanger areas. The technique relies first on experimental data of the phase lag of the outer surface temperature response to periodic heating, and second on a simplified numerical model of the heat exchanger wall to compute the local heat transfer coefficients from the processed data. The IR temperature data processing includes an algorithm for temperature drift compensation, phase synchronization between the periodic heat flux and the measured temperatures, and Single Frequency Discrete Fourier Transformations. The ill-posed inverse heat conduction problem of deriving a surface map of heat transfer coefficients from the phase-lag data is solved with a complex number finite-difference method applied to the heat exchanger wall. The relation between the local and the mean heat transfer coefficients is illuminated, calculation procedures based on the thermal boundary conditions are given. The results from measurements on a plate heat exchanger are presented, along with measurements conducted on pipe flow for validation. The results show high-resolution surface maps of the heat transfer coefficients for a chevron-type plate for three turbulent Reynolds numbers, including a promising approach of visualizing the flow field of the entire plate. The area-integrated values agree well with literature data. CFD calculations with an SST and an EASM–RSM were carried out on a section of a PHE channel. A comparison with the measured data indicates the shortcomings of even advanced turbulence models for the prediction of heat transfer coefficients but confirms the advantages of EASM–RSM in complex flows.     

Mean Temperature Difference for Shell-and Tube Heat Exchangers with Condensing on the Shell Side

Shetl-and-tube heat exchangers with condensing on the shell side usually have a nonlinear heat release curve and a variable shell-side heat transfer coefficient. Using an effective average coolant temperature, it is possible to separate the calculation of the mean temperature difference from the heat transfer coefficient. Thus, it is possible to calculate the weighted mean temperature difference before the heat transfer coefficients are calculated. As a result, the complexity of the calculations is greatly reduced from present incremental procedures. When compared to a detailed incremental calculation of the mean temperature difference, tht maximum error is about 2%. A manual incremental method is also presented, which requires only moderately more work than industrial methods that use average heat transfer coefficients.     

Difference Scheme for Hyperbolic Heat Conduction Equation with Pulsed Heating Boundary

horotctionIt is well known that the basic law of heatconduction is the Fourier law. It has the formq = --k' vT, and one-dimensional heat conduchondifferenhal equation is. The aboveequations are derived from the hypothesis that thevelocity to establish the thermal balance is infinitelygreat. In the modem heat conduchon theory heattransacts in materials in a licited velocity. The factorsto affect the velocity are the thermal propelles of thematerials. In order tO describe this Problem, the sch…     

舰船燃气轮机高压涡轮动叶多维热耦合设计方法

针对舰船燃气轮机复杂高效冷却叶片设计,基于压力修正算法建立冷却叶片一维管网设计方法;通过快速求解可压缩边界层微分方程获得叶片外换热边界,基于参数化的叶片网格生成方法,采用全隐式有限体积的固体导热求解方法,构建了冷却叶片的耦合传热模型,开发了耦合传热计算程序。对某高压涡轮动叶进行多维热耦合设计,确定冷却流路及冷气分布,通过三维气热耦合计算验证了设计方案的可行性,通过对比分析验证了多维热耦合设计方法对主要流通单元的流量、压力误差小于5%,具备较高的工程应用价值。     

A procedure for calculating transient thermal load through multilayer cylindrical structures

This paper presents the transmission matrix and frequency characteristics of transient heat conduction through a multilayer cylindrical structure. The frequency-domain regression method [Appl. Thermal Eng. 21 (6) (2000) 683] is introduced to estimate some simple polynomial s-transfer functions from the frequency characteristics and further calculate the transient heat flow of the cylindrical structure including its thermal response factors and Z-conduction transfer function coefficients. The calculation examples and comparisons have fully demonstrated that the developed procedure is not only easy and simple, but also has high computation accuracy and efficiency.     

The effects of solar chimneys on thermal load mitigation of office buildings under the Japanese climate

The study investigated the performance of a solar chimney, which is integrated into a south facade of a one-story building, as well as the effect on the heating and cooling loads of the building by using a CFD simulation and an analytical model. A C programming code was developed for the calculation of the heating and cooling loads by the heat balance method. The analytical equations of a solar chimney were incorporated into the heat balance calculation. The results showed that the fan shaft power requirement was reduced by about 50% in annual total due to the natural ventilation. It was also found that the solar chimney was beneficial to reduce the heating load by about 20% during the heating season. The annual thermal load mitigation was estimated as 12% by taking the increase of the cooling load into account.     

板坯连铸二冷表面传热系数预测与仿真软件

针对实验室求解表面传热系数以正确制订板坯连铸机二冷区喷水制度的需求，开发了使用修正的直接法求解一维非稳态导热反问题的软件。经校验，软件求解精度高，界面新颖，功能完善，鲁棒性能良好，避免了计算结果的大幅振荡，实用性强且能应用于其他相关领域。     

NUMERICAL CALCULATIONS FOR COMBINED CONDUCTION AND RADIATION TRANSIENT HEAT TRANSFER IN A SEMITRANSPARENT MEDIUM

A numerical computer code was developed for calculating the combined conduction and radiation transient heat transfer in cylindrical, semitransparent materials that have temperature-dependent thermal properties. The radiative component is combined with the equation of conduction heat transfer by adding it as a heat source. The finite element method (FEM) was used for calculating the radiative component and for solving the temperature field in the medium. Very good agreement was observed between results obtained by using our code and those that exist in the literature for several steady-state cases. The advantage of the code is due to the fact that it incorporates temperature-dependent properties; thus it leads to more realistic and accurate results. The code was applied to calculate the cooling path of a large cylindrical sapphire boule while using varying, transient, temperature-dependent, combined heat transfer coefficients.     

含湿非饱和多孔介质的完全分形传热研究

为了探究在含湿情况下多孔介质有效导热率的变化,基于分形理论,考虑多孔介质在含湿时加热过程中相变的影响,结合加热过程中的热量守恒方程和傅里叶导热定律推导出计算有效导热率的新公式。将该模型相关数据代入进行计算,分析了孔隙率、含湿率、面积分形维数和迂曲分形维数对有效导热率的影响。研究发现,孔隙率与有效导热率呈负相关,含湿率与有效导热率呈正相关,分形维数与有效导热率呈负相关。该研究能够反映多孔介质内的传热进程,对于探究微孔结构物质的传热具有一定的指导意义。     

Solar heat gain factors and heat loss coefficients for passive heating concepts

The concept of solar heat gain factor has been introduced for calculating the net energy gain of passive heating elements and other components of a building as a result of incident solar radiation. For passive heating concepts (namely, the direct gain, mass wall, water wall, Trombe wall, and solarium), exact analytical expressions have been obtained for the solar heat gain factors and the corresponding overall heat loss coefficients. These will allow a building designer to calculate immediately the overall heat gain/loss in a building. Numerical calculations have been done for typical values of solar radiation and ambient temperature of typical climatic conditions in India. The method has been compared with the other methods reported in the literature so far. A good comparison is found between the earlier methods and the method of using solar gain factors and the corresponding heat transfer values.     

Thermal analysis of a prototype to determine radiative cooling thermal balance

The paper describes the operation and thermal analysis of a new prototype to determine thermal balance of radiative cooling. The aim of the study is to establish a simple but accurate procedure to calculate the radiative heat exchange between two bodies to be used in the determination of sky temperature, clear sky index or plate emissivity. The radiative transfer calculation has been based on the power required to maintain a constant temperature at the radiative plate of the prototype, provided that the convection and conduction terms are well known. The methodology is applicable to a further advanced system, which avoids convection and reduces conduction to maximise radiative effects, minimising errors and providing more accurate results. Tests have been carried out at a plate temperature of 40 °C, which was optimum for the prototype size and operating conditions. Higher or lower temperatures have lead to a reduction of the fraction of radiative transfer or to the use of very low external power, which complicates the prototype design and makes the temperature control system very costly. Tests carried out in the prototype have shown a perfect matching between total heat transfer and supplied power with an error of less than 5%. This result allows high precision determination of sky temperature, clear sky index or plate emissivity from the use of the proposed methodology.     

Performance Analysis of Heat Sinks With Phase-Change Materials Subjected to Transient and Cyclic Heating

Phase-change cooling technique is a suitable method for thermal management of electronic equipment subjected to transient or cyclic heat loads. The thermal performance of a phase-change based heat sink under cyclic heat load depends on several design parameters, namely, applied heat flux, cooling heat transfer coefficient, thermophysical properties of phase-change materials (PCMs), and physical dimensions of phase-change storage system during melting and freezing processes. A one-dimensional conduction heat transfer model is formulated to evaluate the effectiveness of preliminary design of practical PCM-based energy storage units. In this model, the phase-change process of the PCM is divided into melting and solidification subprocesses, for which separate equations are written. The equations are solved sequentially and an explicit closed-form solution is obtained. The efficacy of analytical model is estimated by comparing with a finite-volume-based numerical solution for both transient and cyclic heat loads.     

基于面热源的钢坯温度分布精确解

将复杂的炉内火焰与钢坯传热过程简化为面热源传热,用数学解析的方法研究钢坯热过程温度响应特征。研究表明:可用非稳态导热数学模型表述钢坯热过程;用分离变量、傅里叶级数、傅里叶变换、线性叠加等方法,求得钢坯温度函数精确解;解析解与FEMLAB3.1数值计算吻合良好,证实了钢坯热过程数学解析的有效性和准确性。     

Monte Carlo method for calculating local configuration factor for the practical case in material processing

The application of Monte Carlo method is discussed to determine configuration factor for the plate including strip elements to two parallel circular cylinders as a case in heating and cooling processes in material processing (e.g. transfer table in hot rolling process). The results show the relationships between different discretization schemes, number of rays used for the configuration factor calculation, and accuracy. Whereas the analytical solutions are not available for this case, Monte Carlo method with 30 and 45 element discretized figures for (100⁴) and (120⁴) rays per element is investigated. The results, obtained from Monte Carlo solution, indicate that smaller elements require more effort to obtain an accurate configuration factor. Additionally, it must be noted that for high accuracy results an increase in the number of rays per element requires the processing time to grow rapidly. By using the configuration factor modeling, the radiative heat transfer can be calculated in various cases for any kinds of heating and cooling processes.     

Modified Wilson Plots for Enhanced Heat Transfer Experiments: Current Status and Future Perspectives

Heat transfer coefficients for enhanced tubes are typically measured indirectly using the “Wilson plot” method to first characterize the thermal performance of the one side (heating or cooling supply) and then to obtain the heat transfer data for the enhanced side based on the Wilson plot results. A brief history of the Wilson plot evolution and alternative methods to the Wilson plot, including the advantages and disadvantages, are discussed as applied for enhanced heat transfer. A slight modification to the Briggs and Young (1969) method is proposed so that the experimental errors can be propagated through the method, allowing us to estimate the error in the generated correlations. Furthermore, a new method based on unconstrained minimization is proposed as an alternative to the least-squares regression. As an example, both methods have been applied to two enhanced boiling tubes (the most recent generation) and heat transfer coefficients were compared against direct wall temperature based heat transfer coefficient measurements made on the same tubes for water flow with high-performance internal helical ribs. Both the unconstrained minimization method and the modified Briggs and Young (1969) method performed well and predicted the same heat transfer performance within experimental uncertainty for two databases taken on two different experimental facilities. Furthermore, if the presently modified Wilson plot method is utilized, and the form of the correlating equation is chosen judiciously and is only applied within the range of experimental conditions tested, the results garnered from the analysis can very adequately predict the local heat transfer performance.     

Parametric study of unsteady flow and heat transfer in a pin-fin heat exchanger

A numerical study has been carried out to analyze the unsteady three-dimensional flow and heat transfer in a parallel-plate channel heat exchanger with in-line arrays of periodically mounted rectangular cylinders (pins) at various Reynolds number and geometrical configurations. The three-dimensional unsteady Navier-Stokes and energy equations are solved using higher order temporal and spatial discretizations. The simulations have been carried out for a range of Reynolds number based on cylinder width (180-600) and a Prandtl number of 6.99 (corresponding to water). Conjugate heat transfer calculations have been employed to account for the conduction in the solid cylinder and convection in the fluid. The thermal performance factor (TPF) increases significantly when the flow becomes unsteady. The choice of aspect ratio of the cylinders is judged by their relative increase in friction factor and heat transfer at transitional Reynolds number. The TPF is found to increase with the increase in pitch of the cylinders. The increase in channel height enhances the TPF though the heat transfer decreases at higher channel height.     

Effect of fixed horizontal louver shading devices on thermal perfomance of building by TRNSYS simulation

Buildings in most countries around the world require large amounts of energy both for cooling and heating. In fact cooling loads due to solar gains represent about half of global cooling loads for residential as well as non-residential buildings. While solar gains through windows contribute largely to these loads, any method of decreasing these gains through shading should be applied with caution, since a balance is required; decreasing cooling loads by shading may increase heating loads drastically and vice versa. So the overall energy requirements both for heating and cooling should be considered. With this in mind a study was done on the thermal performance of a building by TRNSYS simulation, and a shading model for windows was incorporated in it. The shading devices adopted were external fixed horizontal louvers with different slat lengths and tilts. The study was conducted for four different cities in Italy. The optimization of the shading devices was done with respect to primary energy loads for the whole year, and the optimum design was found to depend on location and weather conditions. It was also found that shading factor varies with time of day and is different for summer and winter. For example, for Milan it was found that 70% of gain is cut off in summer, while only 40% is cut off in winter by using optimum shading, which is desirable.     

Thermoeconomic assessment of a multi-engine,multi-heat-pump CCHP (combined cooling,heating and power generation) system – A case study

Design and operation of complex systems for combined cooling, heating and power generation (CCHP) are always a matter of matching performance and demand characteristics of a thermal system set to supply electrical, cooling and heating loads, according to specific usage demands. Equipment selection and operation require the characterization of power, heating and cooling load demands, and their time variation during years, seasons, months and even hours or minutes. The paper aims at utilizing a general model for complex CCHP systems. The proposed model is based on the general theory of exergy cost and structural coefficients of internal links. A general model is presented, and a simple hypothetical cogeneration case is studied. The system operates with two heat engines, with waste heat recovery driving a chiller, in order to meet electrical power and refrigeration loads.