Building Heat Transfer Research Articles

Natural convection in a triangular enclosure due to non-uniform cooling on top

Natural convection in a triangular enclosure subject to non-uniform cooling at the inclined surfaces and uniform heating at the base is investigated numerically. The numerical simulations of the unsteady flows over a range of Rayleigh numbers and aspect ratios are carried out using the Finite Volume Method. Since the upper surface is cooled and the bottom surface is heated, the air flow in the enclosure is potentially unstable to Rayleigh--Benard instability. Based on the numerical results, the transient flow development in the enclosure is classified into three distinct stages: an early stage, a transitional stage, and a steady stage. In addition, the flow inside the enclosure strongly depends on the governing parameters; Rayleigh number and aspect ratio. The asymmetric behaviour of the flow about the geometric centre line is discussed in detailed. The heat transfer through the roof and the ceiling as a form of Nusselt number is also reported. References O. M. Kamiyo, D. Angeli, G. S. Barozzi, M. W. Collins, V. O. S. Olunloyo, S. O. Talabi, A comprehensive review of natural convection in triangular enclosures, Applied Mechanics Reviews , 63 , 060801. doi:10.1115/1.4004290 S. C. Saha, M. M. K. Khan, A review of natural convection and heat transfer in attic-shaped space, Energy and Buildings , 43 , 2564--2571. doi:10.1016/j.enbuild.2011.06.020 V. A. Akinsete, T. A. Coleman, Heat transfer by steady laminar free convection in triangular enclosures, Int. J. Heat Mass Trans. , 25 , 1982, 991--998. doi:10.1016/0017-9310(82)90074-6 E. M. del Campo, M. Sen, E. Ramos, Analysis of laminar natural convection in a triangular enclosure, Numer. Heat Transfer , 13 , 1988, 353--372. doi:10.1080/10407788808913618 H. Asan, L. Namli, Numerical simulation of buoyant flow in a roof of triangular cross-section under winter day boundary conditions, Energy and Buildings 33 , 2001, 753--757. doi:10.1016/S0378-7788(01)00063-9 D. Poulikakos, A. Bejan, The fluid dynamics of an attic space, J. Fluid Mech. 131 , 1983, 251--269. doi:10.1017/S0022112083001317 C. Lei, S. W. Armfield, J. C. Patterson, Unsteady natural convection in a water-filled isosceles triangular enclosure heated from below, Int. J. Heat Mass Trans. 51 , 2008, 2637--2650. doi:10.1016/j.ijheatmasstransfer.2007.09.036 S. C. Saha, J. C. Patterson, C. Lei, Natural convection in attic-shaped spaces subject to sudden and ramp heating boundary conditions, Heat Mass Transfer , 46 , 2010, 1432--1181. doi:10.1007/s00231-010-0607-5 S. C. Saha, Unsteady natural convection in a triangular enclosure under isothermal heating, Energy and Buildings , 43 , 2011a, 695-703. doi:10.1016/j.enbuild.2010.11.014 S. C. Saha, Scaling of free convection heat transfer in a triangular cavity for $Pr > 1$, Energy and Buildings , 43 , 2011b, 2908--2917. doi:10.1016/j.enbuild.2011.07.016 G. A. Holtzman, R. W. Hill, K. S. Bal, Laminar natural convection in isosceles triangular enclosures heated from below and symmetrically cooled from above, J. Heat Transfer , 122 , 2000, 485--491. doi:10.1115/1.1288707 S. C. Saha, J. C. Patterson, C. Lei, Natural convection and heat transfer in attics subject to periodic thermal forcing, Int. J. Thermal Sciences , 49 , 2010, 1899--1910. doi:10.1016/j.ijthermalsci.2010.05.010

System parameter identification theory and uncertainty analysis methods for multi-zone building heat transfer and infiltration

Methods for on-site measurement of building thermal performance system parameters such as coefficient of heat loss, solar heat gain, effective thermal capacity, infiltration rate, and effective mixing volume are very important, yet a nontrivial task. Although these are steady-state parameters, on-site measurements are exposed to changing meteorological conditions and are affected by the thermal capacity of the building. In addition, these parameters should generally be estimated by using a multi-zone model such as inter-zone flow rates. In this regard, a state space equation model, referred to as a “thermal network model,” has been devised to generalize such multi-zone heat transfer system and tracer gas diffusion system measurements. This model is composed of three parameter types, and we have developed a system parameter identification theory and uncertainty analysis method using least squares, as well as actual measurement systems. In the present paper, we improve the least-squares regression equation, the uncertainty analysis method, and the reliability evaluation method. We investigate appropriate excitation waveforms and frequencies for heating and tracer gas release, as well as a low-pass filter for pre-processing measurement data. We verify these theories and methods using computer-simulated measurement.

Optimization method for building envelope design to minimize carbon emissions of building operational energy consumption using orthogonal experimental design (OED)

Climate change has drawn the attention of many researchers and practitioners to focus on the methods to address the challenges in achieving low-carbon buildings and cities and in future developments. Nevertheless, few studies have explored the application of orthogonal experimental design (OED) method to optimize building design for the lowest carbon emissions of building operational energy consumption. This paper aims at developing a new optimization method to building envelope design for the lowest carbon emissions of building operational energy consumption using OED. Firstly, according to the basic principles of the building heat transfer, the method of building’s energy consumption calculation is analyzed, which includes the simplified algorithm based on steady-state heat transfer theory and the dynamic simulation based on the unsteady heat transfer theory. The classification of the building energy consumption influencing factors are then investigated and discussed. Furthermore, based on the basic concept and principle of OED, a case study from the perspective of OED has been conducted, which involves a public building in Nantong, Jiangsu province. Six influence factors of building envelope to building operational energy consumption are selected as follows: exterior wall type, window frame type, glass type, roof type, floor type, and shading type. Selection of optimal case is then determined by the index that is based on the year-round accumulative total load. In the end, Tianzheng Building Energy Consumption Software is used to calculate the optimal case operational energy consumption. The research would provide a new optimization method for building envelope design to the lowest carbon emissions of building operational energy consumption.

VOC Emission from Building Materials in Residential Buildings with Radiant Floor Heating Systems

The emission rates of volatile organic compounds (VOC) from building materials and the resulting indoor concentrations are influenced by temperature. Radiant floor heating systems are widely used in most residential buildings in Korea, and VOC emissions from flooring materials increase as the floor temperature rises. In this study, a numerical model is presented to evaluate the VOC emissions from building materials at different temperatures depending on the heating conditions. A coupled model is developed to simulate the temperature variation of building materials and VOC emissions in the floor heated space, which combines the building heat transfer model for materials and indoor air temperature with the mass transfer model for VOC emissions and indoor VOC concentrations. The model was validated by a mock-up test, and it was then used to examine the emission characteristics from flooring materials under different heating conditions. The results show that emissions of VOC from flooring materials tend to increase as the floor temperature rises during the heating condition.

Co-simulation of building energy and control systems with the Building Controls Virtual Test Bed

This article describes the implementation of the Building Controls Virtual Test Bed (BCVTB). The BCVTB is a software environment that allows connecting different simulation programs to exchange data during the time integration, and that allows conducting hardware in the loop simulation. The software architecture is a modular design based on Ptolemy II, a software environment for design and analysis of heterogeneous systems. Ptolemy II provides a graphical model building environment, synchronizes the exchanged data and visualizes the system evolution during run-time. The BCVTB provides additions to Ptolemy II that allow the run-time coupling of different simulation programs for data exchange, including EnergyPlus, MATLAB, Simulink and the Modelica modelling and simulation environment Dymola. The additions also allow executing system commands, such as a script that executes a Radiance simulation. In this article, the software architecture is presented and the mathematical model used to implement the co-simulation is discussed. The simulation program interface that the BCVTB provides is explained. The article concludes by presenting applications in which different state of the art simulation programs are linked for run-time data exchange. This link allows the use of the simulation program that is best suited for the particular problem to model building heat transfer, HVAC system dynamics and control algorithms, and to compute a solution to the coupled problem using co-simulation.

Heat loss of a trapezoidal cavity absorber for a linear Fresnel reflecting solar concentrator

The present paper studies the heat loss of a linear absorber with a trapezoidal cavity and a set of pipes used for a linear Fresnel reflecting solar concentrator. The study includes the measurements on a 1.4 m long prototype installed in a laboratory, and its thermal simulation in steady-state using EnergyPlus software. Results of the measured vertical temperature variation inside the cavity, the surface interior and exterior wall and window temperatures, the global heat loss at steady-state and the heat loss coefficients, are presented for six different temperatures of the pipes. Measurements revealed a stable thermal gradient in the upper portion of the cavity and a convective zone below it. Around 91% of the heat transferred to outdoors occurs at the bottom transparent window, for a pipe temperature of 200 °C. The heat loss coefficient per area of absorbing pipes ranged from 3.39 W/m 2K to 6.35 W/m 2K (for 110 °C < T pipe < 285 °C), and it increased with the increase of T pipe. Simpler and less time-consuming available free software originally designed for heat transfer in buildings was tested to be a possible replacement of the highly complex CFD software commonly used to simulate the steady-state heat loss of the absorber. The experimental and predicted data sets were found to be in good agreement.

Experimental wooden frame house for the validation of whole building heat and moisture transfer numerical models

An experimental investigation has begun concerning the hygrothermal behaviour of wooden frame houses. The experimental set-up, consisting in a full-scale wooden frame house exposed to natural exterior climate is presented in the paper. It is located in Grenoble (South-East of France), and is equipped with over 150 sensors in order to collect a large amount of data (temperatures, relative humidity). The experimental data presented in the paper enable to get better understanding of coupled hygrothermal phenomena, and can be used to allow the validation of numerical models for heat, air and moisture transfers in wooden frame buildings. The analysed results demonstrate the importance of temperature-driven moisture diffusion from hygroscopic panelling.

Convective Heat Transfer in Rooms with Ceiling Slot Diffusers (RP-1416)

Convection at interior surfaces significantly affects the overall heat transfer in office buildings with large glazing areas. While a large number of these office buildings utilize ceiling slot diffusers at the glazed building perimeter, convection correlations specific to these diffusers have not yet been investigated. This paper describes convection correlations developed for ceiling slot diffusers and examines the effect of temperature, window geometry, and diffuser jet momentum on these correlations. The paper also examines the effect of venetian blinds on the overall correlations. The convective heat transfer coefficients are measured in both heating and cooling conditions. Overall, this paper represents the outcome of over 100 individual full-scale experiments. The results show that forced convection is dominant for a wide range of airflow rates. Correlations are developed as a function of the specific volumetric flow rate of supply air. The temperature of the supply air is also utilized as the reference temperature in convection calculations. The correlations are found to rely primarily on window position and the presence of blinds. The correlations are independent of the temperature difference between surface and supply, diffuser position, and diffuser jet momentum. The paper also quantifies the difference between the convection correlations developed for heated and cooled environments. Finally, utilizing energy modeling to quantify the difference between the new correlations and those previously used, it is found that former convection coefficients underestimate the energy demand. In the modeled building, utilizing the newly developed correlations for forced convection increased the peak cooling load by 24% and the overall energy consumption by 4% over the baseline that had natural convection with constant coefficients at window surfaces.

Two-dimensional hygrothermal transfer in porous building materials

A two-dimensional mathematical model for evaluating the simultaneous heat and moisture migration in porous building materials was proposed. Vapor content and temperature were chosen as the principal driving potentials. The numerical solution was based on the control volume finite difference technique with fully implicit scheme in time. Two validation experiments were developed in this study. The evolution of transient moisture distributions in both one-dimensional and two-dimensional cases was measured. A comparison between experimental results and those obtained by the numerical model proves that they are fully consistent with each other. The model can be easily integrated into a whole building heat, air and moisture transfer model. Another main advantage of the present numerical method lies in the fact that the required moisture transport properties are comparatively simple and easy to determine.

Integrated analysis of whole building heat, air and moisture transfer

There is a continuous dynamic heat, air and moisture (HAM) interaction between the indoor environment, building envelope and mechanical systems. In spite of these interdependences, the current indoor, building envelope and energy analysis tools are used independently. In this paper a holistic HAM model that integrates building envelope enclosures, indoor environment, HVAC systems, and indoor heat and moisture generation mechanisms, and solves simultaneously for the respective design parameters is developed. The model is benchmarked with internationally published test cases that require simultaneous prediction of indoor environmental conditions, building envelope moisture performance and energy efficiency of a building.

Heat transfer to the building structures of the Ignalina NPP accident localisation system

Abstract The performed thermal-hydraulic analysis of the Accident Localisation System (ALS) has shown, that in the case of a Maximum Design Basis Accident (MDBA), in the long term, a significant amount of energy (∼40%) is transferred to the building structures. Therefore, special attention should be paid to the correct simulation of the building structures and heat transfer processes. Free convection, forced convection, condensation and conductivity are the phenomena, which are connected with the heat transfer to the building structures and should be investigated. In this paper, the heat transfer models available in the COCOSYS code, which is applied for the analysis, are discussed. The influence of the heat transfer models on the behaviour of thermal-hydraulic parameters in ALS compartments during MDBA is investigated.

Advanced software tool for the dynamic analysis of heat transfer in buildings; applications to Syria

Abstract The CLIMA computer program is a scientific tool for studying, planning and calculating various buildings (thermal systems), where unusual planning or operating conditions are requested to be taken into account. It was organized to calculate for an optionally determined period within the year. In the CLIMA, the dynamic analysis of heat transfer in buildings is conducted according to the adopted mathematical model by using a one-hour time increment. As a starting point for the calculation of non-stationary heat transfer in a building, the “Room Thermal Balance Method” was adopted. It was organized for use according to the working conditions (holidays, and time of starting/stopping of heating/or refrigeration) and weather seasons (months of winter and summer) in Syria. The computer program was provided with the TMY Database, which was generated in a previous stage of this work. Moreover, it was also modified to be adapted for using the suggested mathematical models, which are derived in the previous stage of this work, as an alternative to the using of TMY database.

Energy Saving by Utilizing Natural Ventilation in Public Housing in Hong Kong

Building occupants can enjoy a healthy and comfortable indoor environment and use less cooling energy if sufficient natural ventilation is available in their dwellings. Assessing the natural ventilation performance of building designs requires modelling of the external wind environment, the natural ventilation rate and the thermal environment in individual rooms, and reduction in the use of air-conditioning equipment. These are complicated processes. A practical approach for assessing the natural ventilation performance of residential building designs is presented in this paper. The method includes prediction of wind pressures upon window openings in the building façade by computational fluid dynamics simulation, natural ventilation rate prediction using a flow network simulation model, and indoor free-float temperature and air-conditioning energy-use predictions using a building heat transfer and an air-conditioner performance simulation programme. Additionally, the method includes a simplified statistical approach to deal with the random variations in the speed and direction of the wind. This method has been applied to assess the natural ventilation performance of a standard public housing block design widely used in Hong Kong, taking into account when the wind wing walls were incorporated into the building façade and when the separation distances among the building blocks were widened by 25% and 50%.

Simulation of coupled heat and moisture transfer in air-conditioned buildings

The simultaneous heat and moisture transfer in the building envelope has an important influence on the indoor environment and the overall performance of buildings. In this paper, a model for predicting whole building heat and moisture transfer was presented. Both heat and moisture transfer in the building envelope and indoor air were simultaneously considered; their interactions were modeled. The coupled model takes into account most of the main hygrothermal effects in buildings. The coupled system model was implemented in MATLAB-Simulink, and validated by using a series of published testing tools. The new program was applied to investigate the moisture transfer effect on indoor air humidity and building energy consumption under different climates. The results show that the use of more detailed simulation routines can result in improvements to the building's design for energy optimisation through the choice of proper hygroscopic materials, which would not be indicated by simpler calculation techniques.

Methodologies for shortening test period of coupled heat-moisture transfer in building envelopes

Moisture movement and condensation through the building envelope can cause serious problems including reduction in thermal resistance of the wall, mold growth, and deterioration of the structural integrity of the wall. Full-scale tests are well suited to study these phenomena. However, a full-scale hygrothermal test requires a long testing period, typically from several months to a year, to study both wetting and drying processes. This paper presents methodologies whereby the test period is shortened to reduce cost and make large scale testing of building envelopes more affordable. A computer program for building heat-moisture simulation and test planning (BHMSTP) is developed using implicit finite-difference equations of coupled heat and moisture transfer in buildings. Two methodologies are proposed based on the analysis with the BHMSTP, shortening the test cycle from a year to a few months. In one method, indoor conditions are varied while maintaining climatic conditions constant; in the other method, weather conditions are varied, while indoor conditions are kept constant. The implementation of methodologies with the BHMSTP is demonstrated using Montreal (Canada) weather data.

From model validation to production of reference simulations: how to increase reliability and applicability of building and HVAC simulation models

Validation of simulation models appears from a long time as a key issue in order to promote a more intensive and more efficient use of simulation models in the field of building and HVAC simulation. IEA Annex 34/43 originally targeted a number of specific applications where a more advanced validation was required: ground coupling problems, multizone building, shading, day lighting and cooling load interaction, HVAC components and ventilated facades. These validation exercises were built on the large methodological experience obtained in previous projects and address sometimes very fundamental problems of heat transfer in buildings. Consulting engineers and practitioners might see these exercises as a bit too far from their objectives and it is the reason why an additional activity was proposed with the specific aim of producing, based upon the results of the validation of models, a set of reference simulations. These applications cover a range of building types (residential, commercial) and systems (production, distribution emission) and run in a variety of climates. The paper will describe how models dedicated to these applications were developed, starting from validation results, going through the selection and consolidation of simulation hypothesises and ending with a number that might be considered as reference for the concerned applications. The paper will concentrate on models required by a residential building application (multizone building equipped with a heat pump or a condensing boiler, heat emitted by radiators or floor heating systems). Simulations make use of both EES and TRNSYS software and both software are applied in parallel as far as possible in the different applications in order to get a better judgment of their potential advantages and drawbacks. The use of reference simulations in view of qualifying normative methods currently in development in the frame of the European Energy Performance in Buildings Directive is also addressed and demonstrated in the paper.

Experimental and theoretical investigation of non-isothermal transfer in hygroscopic building materials

In this paper a modified two-dimensional Luikov model for evaluating the non-isothermal moisture migration in porous building materials was proposed. The coupled heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as the principal driving potentials. The coupled equations were solved by a numerical method, which consists of a finite difference technique with a fully implicit scheme in time. Two validation experiments were developed in this study. The evolution of transient moisture distributions in both one-dimensional and two-dimensional cases was measured. A comparison between experimental results and those obtained by the numerical model proves that they are fully consistent with each other. The modified model can be integrated into a whole building heat, air and moisture transfer model.

Dynamic earth-contact building: A sustainable low-energy technology

Sustainable development includes low-energy buildings, which reduce energy consumption, green house gases emission, water usage, etc. The choice of subsurface wall at varying depths for construction of buildings has a direct impact on energy consumption and the environment. This paper includes in its scope all building structures in which a significant area is in direct contact with the earth, and excludes indirect earth contact. A two-dimensional simplified analytical Fourier boundary series was developed to study dynamic earth-contact heat transfer and humidity variation in building. It was used in conjunction with the whole-building energy simulation program developed in the TRNSYS environment. The predictions were compared with extensive experimental data measured from typical existing earth-contact buildings situated in the Indian Institute of Technology Delhi campus. Heat transfer through earth-contact building structures was predicted to decrease with increasing soil contact. The results obtained from the detailed model showed that earth-contact structures form excellent passive technology that can be exploited for energy conservation. This work is part of an effort to develop zero energy building models in India. The presented model can be easily incorporated into indirect earth-contact structures in order to describe the impact of cooling tubes, earth-air tunnels or heat pumps in indoor environment.

Effect of Solar-Cycle on Terrestrial Weather Parameters at Ibadan, Nigeria

Investigations have been made of the morphology of the sun-weather relationships in terms of possible solar cycle effects on terrestrial weather parameters. The study was done using data from measurements carried out at Ibadan (lat 7.4°N, long 3.9°E) Nigeria during solar cycle 21 (1980–1990) and solar cycle 22 (1991–2001). It has been found from the full correlation analysis that the magnitudes of the various weather parameters are not dependent on solar activity. The results of the study unfold significant features of the sun-weather relationship at the studied station. The implications of the investigation on building design guidelines and heat transfer models of building components are also discussed.

ZAER: A Zonal Model for Heat Transfer and Air Flow in Unconditioned Buildings. - An Experimental Validation

This paper presents a three-dimensional zonal model, ZAER, for heat transfer and air flow calculations. It is based on an intermediate approach between single-air-node and CFD models. The indoor air volume is divided into macroscopic homogeneous zones. Heat and mass balance equations are written for each zone, while the mass flow rates across the interfaces are calculated by power pressure laws. The simulation tool ZAER allows the determination of temperature fields and air flow distributions inside unconditioned buildings, taking into account external boundary conditions. We also present an experimental validation of the model by comparing its predictions with experimental data obtained from measurements on the experimental cell Minibat (CETHIL, INSA Lyon Laboratory), for different configurations. The results obtained are in good agreement with experimental data, and suggest that the ZAER model is an appropriate tool to assess satisfactorily temperature heterogeneity and air movement within a room.