Transfer function coefficients for time varying coupled heat transfers in vertically heated hollow concrete bricks

This paper describes a two step numerical procedure to determinate empirical transfer function coefficients (TFCs) for vertically heated hollow concrete bricks. For such systems TFCs cannot be generated using the analytical techniques available in the literature such as the z-transfer function method or the space state representation method because of the nonlinear local character of the heat transfer by natural convection and radiation in the air cells of the hollow concrete bricks. The first step of the procedure consists in predicting coupled heat transfer by conduction, convection, and radiation in realistic time varying conditions using a detailed numerical simulation. In the second step, the results of the simulation (the time-varying heat fluxes at the hollow brick surfaces) are used to obtain empirical transfer function coefficients using an identification technique. Transfer function coefficients are generated for three different types of hollow concrete bricks mostly used in practice. It is shown that the empirical transfer function coefficients permit fast and accurate prediction of heat transfer for thermal excitations that differ markedly from those used to generate these coefficients without solving the complex system of equations governing the coupled heat transfer mechanisms.     

空心砌块的传导传递函数系数计算与验证

根据空心砌块的频域热特性辨识其s多项式传递函数,并进一步求取空心砌块的CTF系数。根据空心砌块动态热特性实验结果对采用CTF系数计算空心砌块热特性的可靠性进行了验证。结果表明,在边界条件趋于周期性稳定后,采用CTF系数和实验得到的空心砌块内外表面温度曲线非常吻合。因此,CTF系数能准确的计算空心砌块的动态热特性,是一种分析空心砌块动态热特性的有效方法。     

Numerical simulation by the FVM of coupled heat transfers by conduction, natural convection and radiation in honeycomb’s hollow bricks

This paper presents a detailed numerical study, in steady state regime, of the interaction between two dimensional heat transfers by conduction, natural convection and radiation in double hollow bricks formed by two honeycomb walls separated by an air layer. The air motion in all cavities of the system is laminar. The left and right vertical sides of the hollow bricks are considered isothermal and maintained at different constant temperatures. The top and bottom horizontal sides are assumed to be adiabatic. The governing equations are solved using the finite volume method (FVM) and the SIMPLE algorithm. The impact of the thickness of the air layer on the global heat flux through the structure is discussed. The simulation results show that the variation of the overall heat flux through each hollow brick as a function of the temperature difference ΔT between the vertical sides of the system is almost linear for the different types of double hollow bricks considered. This linear thermal behaviour allowed the generation of appropriate overall heat exchange coefficients that permit fast and accurate prediction of heat transfers through the hollow bricks without solving the complex system of equations governing the coupled heat transfers. Comparison of the performance of different types of double hollow bricks is made.     

Coupled heat transfer modes for calculation of cooling load through hollow concrete building walls

A CFD model was developed to study thermal performance of hollow cement wall constructions of buildings under hot summer conditions. The approach employed couples conjugate, laminar natural convective flow of a viscous fluid in hollow building blocks with long-wave radiation between the cavity sides. Realistic boundary conditions were employed at the outdoor and indoor surfaces of the wall. A state-of-art building energy simulation program, ESP-r, was used to determine the outdoor thermal environment that included solar radiation, equivalent temperature of the surroundings and convective heat transfer coefficient. The CFD problem is put into dimensionless formulation and solved numerically by means of the control-volume approach. The study yielded comprehensive, detailed quantitative estimates of temperature, stream function and heat flux throughout the wall domain. A detailed parametric study showed that using a wider cavity within a building block does not necessarily reduce heat flux through the block. Radiation heat transfer between cavity sides may account for a significant fraction of heat flux through the block and neglecting its effect can lead to errors that could be as large as 46%. The geometry of the hollow blocks was demonstrated to affect the heat flux by as much as 30%.     

A frequency-domain regression method for estimating building foundation heat transfer

In this article, the Interzone Temperature Profile Estimation solutions are first utilized to carry out a frequency analysis for the foundation heat transfer to determine the effect of indoor air and ambient air temperature fluctuations on the variation of the foundation heat loss/gain. Then, a frequency-domain regression-based method is used to develop transfer functions for ground-coupled surfaces. The regression frequency-domain method is tested and validated using one-dimensional heat transfer for above-grade walls. It is found that the proposed method provides a more effective alternative than numerical methods to estimate conduction transfer function coefficients for building foundations and thus can be easily implemented in whole-building simulation programs.     

Verification for transient heat conduction calculation of multilayer building constructions

Validation and verification of building simulation programs and load calculation programs is of continuing interest. Dynamic thermal behavior data, including conduction transfer function (CTF) coefficients, thermal response factors and periodic response factors, are used to calculate transient heat conduction through building constructions. Computational inaccuracy sometimes occurs in calculating CTF coefficients and response factors. In this paper, a method for verification of the CTF coefficients and response factors over the whole frequency range is introduced. This method is based on the equivalence of dynamic models for a linear system and the frequency characteristics of building transient heat transfer models. Bode diagrams and error criteria are proposed to verify the CTF coefficients and response factors. Some examples are given to demonstrate the methodology.     

多层墙体热湿耦合传递模型及验证

从非平衡热力学角度论证了多层墙体热湿耦合过程采用水蒸气分压力和温度作为驱动势的合理性。由于水蒸气分压力是含湿量和温度的函数,利用全微分思想,建立了多层墙体热湿耦合传递模型,该方法可避免Budaiwi方法在热湿耦合模型建立过程中采用的空气含湿量与相对湿度间的近似表达式,而且简化了方程系数,便于方程的求解。通过对多层墙体求解结果的对比,验证了该模型的有效性。     

Analysis of coupled natural convection–conduction effects on the heat transport through hollow building blocks

In the present study, the coupled convective and conduction heat transport mode in a common hollow building brick is studied. Heat transfer rate through building bricks is examined in order to asses the suitable brick insulation configuration. Three different configurations for building bricks are considered. The first is a typical brick of three identical hollow cells (air cavities), the second is obtained by filling these cells with ordinary polystyrene bars and the third is obtained by using hollow polystyrene bars. The geometry of the first and third configurations considered in this study is simply a solid closed frame surrounding square cavities filled with air. The second configuration is a solid composite slab. Solving Navier–Stocks equations assuming Boussinesq approximation, using the commercial Fluent software, showed that the cellular air motion inside blocks’ cavities contributes significantly to the heat loads. Insertion of polystyrene bars reduced the heat rate by a maximum of 36%. Using a hollow polystyrene bars reduces the heat rate by 6% only due to the air motion inside cells. In order to estimate the heat rate during a day, the air temperature and solar insolation data of a typical summer day for the city of Jeddah, Saudi Arab, are used. A quazi-steady state approach is implemented to estimate an equivalent facade surface temperature, which is then used as boundary for solving the simulation model. Such an approach showed that the effective overall daily heat rate reduction using polystyrene filled bricks to be 25%.     

A simple method for estimating transient heat transfer in slab-on-ground floors

The problem of calculating transient heat transfer in concrete floor slabs is complicated due to ground coupling, which can require the numerical solution of two or three-dimensional transient conduction equations. This paper presents a simplified method for calculating transient slab-on-ground heat transfer that can be incorporated within hourly simulation programs. The method assumes that there are two primary one-dimensional paths for heat transfer from a ground-coupled floor slab: (1) one-dimensional heat transfer from the perimeter of the slab to the ambient and (2) one-dimensional heat transfer between the slab interior surface and a portion of the soil beneath the slab. The perimeter heat transfer is assumed to occur at quasi-steady state and is characterized in terms of a perimeter heat loss factor (F_p). Transient heat transfer within the slab and ground are modeled using a simple thermal circuit employing three nodes with an adiabatic boundary condition at a specified depth within the soil underneath the slab. Although some simulation models consider this type of two-path model, there appears to be no validation of this approach and there is no guidance for specifying perimeter heat loss factors and underfloor soil depths and node locations for the thermal circuit. In the current paper, results from detailed two-dimensional finite-element models for typical floor constructions and soil properties were used to identify (1) locations for nodes within the slab and soil, (2) correlations for soil depth as a function of soil properties associated with the underfloor adiabatic boundary condition, and (3) correlations for perimeter heat loss factor as a function of soil properties and edge insulation levels for different constructions. Transient heat transfer results from the simple model compared well with results from the finite-element program for different floor constructions, edge insulation, soil properties, locations, and times of year.     

Optimisation of ceramic heat storage of stoves

In order to optimise the efficiency of tile stoves with a thermal capacity of up to 10 kW using wood logs, an important task is to investigate the heat storage of such firing systems used for space heating. For a basic understanding of heat transfer and heat storage behaviour, experiments as well as numerical simulations were carried out. Within the framework of these investigations, two different types of tile stoves were tested: a single-walled stove without an air gap and a double-walled stove with an air gap. Findings show that the air gap influences the heat storage behaviour causing a smooth surface temperature distribution and a decline in efficiency on the other hand. In order to avoid this negative effect, variations of the width of the air gap, ceramic mass, material and length of the flue gas tube were carried out. To remedy the deficiencies of the double-walled design, an extension of the length of the flue gas tube by 10–14% is necessary. Dense chamotte also allow double-walled tile stoves with the same efficiency as single-walled stoves using standard chamotte.     

Empirical validation of different internal superficial heat transfer models on a full-scale passive house

Being highly insulated, low-energy buildings are very sensitive to variable solar and internal gains. In this context, some modelling assumptions frequently used in simplified building energy simulation tools might be called into question. While higher insulation levels reduce the influence of heat transmission through opaque walls, absorption of solar and internal gains at inner wall surfaces, and indoor superficial heat transfers, become concerning. The convective and long-wave radiative heat transfer models are investigated in COMFIE, a dynamic energy simulation platform. More detailed internal heat transfer models are developed by decoupling convective and long-wave radiative heat transfers and using time-dependent coefficients. Furthermore, an empirical validation process on both simplified and detailed models is carried out using measurements from a full-scale experimental concrete passive house, addressing the model uncertainty vs. complexity issue.     

R134a冷水机组干式蒸发器两种模型比较

冷水机组干式蒸发器通常采用内螺纹强化传热管,由于内螺纹管的几何结构比光滑管复杂,故很难准确预测制冷工质在内螺纹管内的局部沸腾换热特征。对于干式蒸发器的设计,必须要知道在设计工况下的管内流动沸腾换热系数。因此本文采用两种换热模型,对比分析R134a在内螺纹管内的沸腾换热特性,从而为R134a冷水机组干式蒸发器的设计提供理论上的指导。研究表明,这两种换热模型的计算结果都在合理的范围内,而RinYun模型可以用来计算不同干度区域的局部沸腾换热系数,且该模型考虑的影响因子较为全面。     

空心砖瞬态传热数值研究

利用自然对流和热传导的物理耦合模型,控制方程采用有限容积法,求解算法使用SIMPLE算法,研究了空心砖在外界环境变化条件下的瞬态传热规律。通过计算得到空心砖内外壁温的变化情况及延迟特性,同时也研究了通过空心砖的热流变化规律,并与实心砖的传热过程比较,发现实心砖和空心砖的内壁面温度的延迟相差不大。而空心砖与实心砖热流量相差很大,实心砖的平均热流约比空心砖平均热流大41.5%,说明空心砖节能效果显著。     

多元平行流蒸发器空气侧百叶窗翅片的数值模拟

对多元平行流蒸发器空气侧百叶窗翅片流动和传热进行了数值模拟。得到了不同迎面风速下的空气温度场、压力场、翅片表面局部换热系数。计算得出的空气侧换热系数和压降与实验关联式一致。分析研究了翅片间距和百叶窗角度对空气侧传热和阻力特性的影响,研究结果对多元平行流蒸发器的设计和优化有重要的指导意义。     

空心管电加热器壳侧强化传热性能模拟

空心管电加热器是一种依靠传热管本身作为发热元件来加热流体的新形式。本文提出的改进方案,使壳侧流体纵向流动,消除了流动死区。充分利用管内外加热空间强化传热。对不开孔,逆流,顺流和分流四种形式的电加热器进行了数值模拟。结果表明分流方案减小了固定管板两侧热应力,壳侧综合指标最高,壳侧压降最小,是最佳改进方案。     

Simulation of buoyancy-driven natural ventilation of buildings—Impact of computational domain

Two computational domains have been used for simulation of buoyancy-driven natural ventilation in vertical cavities for different total heat fluxes and wall heat distributions. Results were compared between cavities with horizontal and vertical inlets. The predicted ventilation rate and heat transfer coefficient have been found to depend on the domain size and inlet position as well as the cavity size and heat distribution ratio. The difference in the predicted ventilation rate or heat transfer coefficient using two domains is generally larger for wider cavities with asymmetrical heating and is also larger for ventilation cavities with a horizontal inlet than those with a vertical inlet. The difference in the heat transfer coefficient is generally less than that in the ventilation rate. In addition, a ventilation cavity with symmetrical heating has a higher ventilation rate but generally lower heat transfer coefficient than does an asymmetrically heated cavity. A computational domain larger than the physical size should be used for accurate prediction of the flow rate and heat transfer in ventilation cavities or naturally ventilated buildings with large openings, particularly with multiple inlets and outlets. This is demonstrated with two examples for natural ventilation of buildings.     

Detailed numerical simulation of coupled heat transfer by conduction,natural convection and radiation through double honeycomb walls

The aim of the present work is to study numerically 2-D steady state coupled heat transfer by conduction, free convection and infra-red radiation through two honeycomb walls separated by a vertical air layer. Airflow in both holes and separating air layer is laminar. The limiting vertical sides of the double honeycomb wall are assumed to be isothermal but at different temperatures while the upper and lower horizontal surfaces of the structure are insulated. The FVM method and the SIMPLE algorithm are used to solve numerically the equations of conservation of mass, momentum and energy in both air filled cavities and solid partitions. It is found that the global heat flux across the entire wall varies almost linearly with the difference between the outside and the inside temperatures. Based on this linear thermal behaviour, appropriate overall heat exchange coefficients are derived. These coefficients can be used easily in practice to predict the global heat transfer across the studied honeycomb walls without solving the detailed and complex equations that govern the different heat transfer mechanisms. Effect of the thermal conductivity of the construction material on the overall heat transfer through double honeycomb walls is studied.     

Fast simulation of dynamic heat transfer through building envelope via model order reduction

In this paper, a fast and accurate numerical simulation method on dynamic heat transfer through building envelopes has been developed by using the Krylov subspace and the balanced truncation model order reduction (MOR) algorithms. The computational accuracy and efficiency of the two MOR algorithms are discussed through the numerical simulation on a roof heat transfer in a one-day period, and then the two verified algorithms are applied to simulate the heat transfer through a multilayer wall for a week and the two-dimensional heat transfer through an L-shape thermal bridge. The results show that the relative errors of the two algorithms to the harmonic response method or to the direct solution method are all less than 1%, and the solving time with the two MOR algorithms decreases greatly. In addition, the Krylov subspace MOR algorithm has a faster solving speed and is more suitable for solving the heat transfer through a building envelope than the balanced truncation MOR algorithm.     

Single thermal zone balance solved by Transfer Function Method

We present an algorithm that uses the Z-transform operator to face the problem of heat transmission in a single thermal zone composed by multilayered walls. The method is very flexible and could be adopted to calculate the transfer function coefficients able to simulate the thermal behaviour of a room in free floating. Knowing the transfer function coefficients, it is possible to simulate the dynamic profile of each inner surfaces temperature and furthermore of the inner air temperature.The proposed algorithm is fully described granting maximum clarity. The explicitness of all steps of the calculus make possible the definition of a method that is able to vary all of the calculus parameters such as sampling g period, number of roots, number of poles or number of coefficients.To assess the reliability of the algorithm, we carried out a comparison between simulation data obtained from our method, from Fourier steady-state algorithm and those obtained from TRNSYS.     

Natural convection at an indoor glazing surface with different window blinds

In the present study, an empirical model to determinate the convective heat loss, at an indoor glazing surface, is proposed. This model allows calculating the convective heat transfer coefficient and the air flow rate entering to the window cavity formed between the glazing surface and the protection device. The window blind is first studied experimentally by using a rigid paper, which is installed at four different distances from the window frame. This configuration is used as reference to determinate a global model, which is mainly composed of two correlations: one for the Nusselt number and other one for the air mass flow rate incoming to the window cavity. Then, more realistic configurations are tested: single curtains, double curtains, PVC blinds, wood blinds, Venetian blinds or polyester blinds. In general, heat transfer coefficients for these configurations are equal or higher than that obtained with a free plate. Several correlations are proposed for each configuration.