Convective Heat Transfer Coefficient Correlations Research Articles

Wind-tunnel experiments on cross-ventilative cooling in a generic isolated building with one heated wall: Impact of opening size

This paper presents wind-tunnel experiments of cross-ventilative cooling in a generic isolated building with an interior heated side wall. Two different sizes of openings are considered: large and small openings. Particle image velocimetry (PIV) is used to determine velocities in the vertical centerplane. Air temperatures in the vertical centerplane are measured using negative temperature coefficient (NTC) sensors. Surface temperatures on the heated wall are measured using an infrared camera. Surface heat fluxes are obtained using heat flux sensors. In both cases the indoor airflow is dominated by the jet through the openings, with higher velocities in the building with large openings. The air temperatures measured with small openings are up to 7.5 % larger than those with large openings. The surface heat fluxes are up to 20 % higher in the building with large openings. The interior convective heat transfer coefficients vary considerably across the heated wall for both opening sizes and can be very different (up to 5 times higher) from those obtained by existing internal convective heat transfer coefficient correlations. The measurement results give insight into the complexity of ventilative cooling and can be used to validate computational fluid dynamics (CFD) simulations of cross-ventilative cooling.

Impact of human body shape on forced convection heat transfer

Predicting human thermal comfort and safety requires quantitative knowledge of the convective heat transfer between the body and its surrounding. So far, convective heat transfer coefficient correlations have been based only upon measurements or simulations of the average body shape of an adult. To address this knowledge gap, here we quantify the impact of adult human body shape on forced convection. To do this, we generated fifty three-dimensional human body meshes covering 1st to 99th percentile variation in height and body mass index (BMI) of the USA adult population. We developed a coupled turbulent flow and convective heat transfer simulation and benchmarked it in the 0.5 to 2.5m·s-1 air speed range against prior literature. We computed the overall heat transfer coefficients, hoverall, for the manikins for representative airflow with 2m·s-1 uniform speed and 5% turbulence intensity. We found that hoverall varied only between 19.9 and 23.2 W·m-2K-1. Within this small range, the height of the manikins had negligible impact while an increase in the BMI led to a nearly linear decrease of the hoverall. Evaluation of the local coefficients revealed that those also nearly linearly decreased with BMI, which correlated to an inversely proportional local area (i.e., cross-sectional dimension) increase. Since even the most considerable difference that exists between 1st and 99th percentile BMI manikins is less than 15% of hoverall of the average manikin, it can be concluded that the impact of the human body shape on the convective heat transfer is minor.

Experimental investigation of mixed and forced convection generated by displacement ventilation with radiant wall heating

There is a rising necessity concerning low energy consumption and thermal comfort, thus, new systems combining displacement ventilation and radiant heating are becoming common as a research topic. The current paper covers free, mixed, and forced convection in an experimental chamber consisting of a radiant wall heating system and a baseboard level diffuser. Air temperature in the test room is adjusted using an air blower via a novel designed baseboard level slot diffuser along with a hydronic wall heating circuit. Simultaneously; an upward direction air jet is blown through the heated or the opposite wall under two scenarios. The ranges of air inlet velocity and temperature are selected to be between 0.25 and 12.5 m/s and 14–26 °C, correspondingly. The heat transfer characteristics pertaining to the heated wall are studied with respect to the influences of wall and air temperatures, air inlet velocities, and the location of the diffuser. To derive convective heat transfer coefficient correlations valid for mixed and forced types for a radiant heated wall, acquired data have been processed. The convective, radiative, and total heat transfer coefficient intervals of 3–14.1, 5.2–5.5, and 9.3–21.7 W/m2K have been obtained for the radiant heated wall coupled with airflows, correspondingly.

An experimental study on the heat transfer characteristics over a radiant cooled wall exposed to mixed and forced convection driven by displacement ventilation

An increasing demand on energy efficiency and indoor air quality exists, therefore displacement ventilation and radiant surface cooling applications are taking more attention as an investigation subject. This article presents an experimental study of mixed convection in a test chamber equipped with both radiant wall cooling and air-conditioning systems. The room is conditioned by a hydronic circuit with embedded pipes in the wall and an air jet supplied by a newly designed baseboard longitudinal slot that is apart from relevant literature. The tested radiant wall temperature is kept around 14 °C. Meantime; a parallel upward directional air jet is applied over the cooled wall or the opposite wall. The air jet velocity and its temperature lie within the intervals of 0.25–10 m/s and 18–28 °C, respectively. The natural, mixed, and forced convection heat transfer characteristics over the wall have been investigated considering the effects of radiant cooling, airflow rates, and ventilation location. The measured data are used for the derivation of mixed and forced average convective heat transfer coefficients correlations along a cooled wall. Consequently, the convection and total heat transfer coefficients over the radiant cooled wall are calculated in the ranges of 2–7.3 W/m2K, and 9.1–13.9 W/m2K, respectively.

Simulation and optimization of night cooling with diffuse ceiling ventilation and mixing ventilation in a cold climate

Accurate convective heat transfer coefficient (CHTC) is crucial to assess the night ventilation performance accurately. Previous experimental studies proposed CHTC correlations tailored for night cooling with diffuse ceiling ventilation (DCV) and mixing ventilation (MV). A holistic approach integrating the building energy simulation and optimization method was put forward to validate the experimentally proposed correlations, simulate and optimize the performance of night cooling with DCV and MV. The validation results demonstrated that the proposed correlations predicted the surface temperatures accurately with a maximum mean absolute error of 1.9 °C. For a typical air-conditioned office room, night cooling with DCV saved about 0.2 kWh/m2 total cooling energy per unit floor area (TCEC) more and provided a lower average predicted percentage of dissatisfied during the working hours (aPPD) by up to 3.1% than that with MV. Compared with the validated correlations, the empirical ceiling diffuser convection algorithm overestimated the night energy-saving potential by up to 0.5 kWh/m2 (13.8%) and yielded a maximum difference in the aPPD of 1.5%. Optimization significantly saved TCEC by 1.1 kWh/m2 (29.0%) for DCV and 0.9 kWh/m2 (25.7%) for MV while maintaining the aPPD within 10%, compared to respective base cases.

Thermal zoning and window optimization framework for high-rise buildings

Window sizing and configuration can have a significant influence on building energy consumption. Window selection often has a conflicting objective on heating, cooling, and lighting performance. The smaller window performs better on controlling heat loss in winter and solar heat gain in summer, while the larger window performs better on providing views, daylight, and solar heat gains in winter. Also, the energy consumption analyses of high-rise buildings have some fundamental limitations that include the changes in microclimate parameters with altitude, the treatment of building size, uncertainties associated with the existing convective heat transfer coefficients correlations (CHTC). This study provides a framework for simulation-based optimization of window configuration for a high-rise building to minimize its energy consumption. The technique involves CFD modeling to validate and develop new-CHTCs, a Building Energy Simulation used to assess the energy consumption using the newly developed CHTC, and a numerical optimizer for iterative optimal window configuration selection. The decision parameters are window size and room location. The thermal comfort temperature set points and daylight illuminance are taken as constraints. The proposed approach is implemented as a case study on a single by single room model basis positioned at different heights in an isolated 100 m tall building exposed to Boston, MA microclimate. For a room located on the 2nd, 15th, and 29th floor, an optimum window configuration of 30%, 48%, and 30%, window-to-wall ratio, respectively, are obtained.

Experimental investigation of convective heat transfer for night ventilation in case of mixing ventilation

The purpose of night ventilation (NV) is to improve building energy and thermal comfort performance. The key to accurately predicting NV performance is selecting the appropriate convective heat transfer coefficient (CHTC) at different surfaces of the built environment. The current CHTC correlations used in building energy simulation tools are limited and prevent accurate modeling of NV strategies. The building thermal mass activation is also an important factor to influence the efficiency of NV. A series of dynamic full-scale experiments with ten thermal mass distribution schemes, four air change rates per hour (ACH), and two inlet air temperatures was conducted to derive the CHTCs at test room surfaces during NV in case of mixing ventilation. The results show that the existing correlations did not accurately predict the CHTC for most cases. Therefore, the new surface-averaged CHTC correlations with inlet temperature as reference were developed for different thermal mass distribution schemes and ACH. Installing the thermal mass on one surface can significantly enhance its CHTC and affect the CHTC at other surfaces. The mean CHTC of the test room was independent of the inlet air temperature but increased together with ACH and thermal mass level of interior surfaces. Finally, the presence and locations of tables had limited influence on the CHTC at interior surfaces but reduced the mean CHTC.

Effect of localized exterior convective heat transfer on high-rise building energy consumption

The energy consumption analyses of high-rise buildings have some fundamental limitations that include the treatment of building size, changes in microclimate parameters with altitude, and the uncertainties associated with the existing building facade convective heat transfer coefficients correlations (CHTC). This study investigates the effects of these parameters on the energy consumption by individual rooms at a different location as part of a 100 m high-rise building, exposed to different weather conditions, having a different window-to-wall ratio. In the first part of the study, representative new-CHTC at the windward facade of the building is generated by using CFD simulations. In the second part of the study, comparative energy consumption assessment is carried out using the newly generated CHTC and other commonly used correlations by using EnergyPlus. The result shows that for high-rise building with 100% WWR exposed to a windy microclimate (such as Boston, MA), a deviation of 11.2% and 4.7% on annual heating and cooling energy consumption, respectively, have been observed. Further, the energy consumption of each room throughout the building height was also investigated to investigate the effect of building height. Compared to a room located at the mid-height (15th floor), the annual heating consumption on a room located at the 5th floor room was 7.8% lower and a room located on the 25th floor room the consumption was 7.6% higher. In summary, this study highlights the importance of accurate local CHTC generation to enhance thermal comfort in individual rooms and to optimize overall building energy consumption.

Numerical and Experimental Calculation of CHTC in an Oil-Based Shaft Cooling System for a High-Speed High-Power PMSM

The convective heat transfer coefficient (CHTC) is a critical parameter that is required for developing an accurate and efficient thermal design of electrical machines. However, the existing empirical CHTC correlations are invalid for an oil-cooled hollow-shaft rotor. On this basis, a simplified numerical model based on computational fluid dynamics methods is developed in this paper to provide a qualitative understanding of the rotational effects on the convective heat transfer across a range of operation speeds. Then experiments are undertaken to validate the data obtained from numerical models and to estimate the impact parameters on the CHTC, such as the rotational velocity, coolant flow rate, and coolant temperature. On the basis of the numerical and the experimental results, it is concluded that the rotation can significantly increase the CHTC of the shaft inner wall surface above the level of the stationary condition. However, the axial flow rate and the viscosity of the coolant have less influence on convective heat transfer for the high rotational speeds. As a result of such analysis, a general correlation is defined by using Nusselt numbers as a function of rotational Reynolds numbers and Prandtl numbers.

Influence of the façades convective heat transfer coefficients on the thermal energy demand for an urban street canyon building

In an urban micro-climate environment, the convective heat transfer coefficient (CHTC) on the façades influences simulated building's energy demand and exterior wall surfaces temperatures. In this paper, it is analyzed how the CHTC values on the façades of a building located in an urban canyon influence the façades temperatures and how important is the choice of an accurate CHTC correlation on the space cooling and heating energy demand. CHTC correlations found in literature are based on some specific micro-climate parameters such as local wind speed, district construction density, temperature differences between façades and canyon air and wind direction. An accurate choice of the right correlation for the simulated urban environment is important to better represent the exterior walls heat removal due to outside wind climate. The effects of the use of different CHTC correlations have been evaluated by means of TRNSYS 17.0 simulation program. The study is performed for a building sited an urban street canyon with the aspect ratio H/W=1 and located in a Mediterranean climate, in Rome. The comparison performed between the results of the numerical simulations shows that some correlations lead to an underestimation of the space heating demand around 9.7% and to an overestimation of the space cooling demand around 17.5%.

A simplified experimentally tested theoretical model to reduce water consumption of a direct evaporative cooler for dry climates

The aim of this study is to introduce a simple modified experimentally tested theoretical model to mainly reduce the water consumption rate of a direct evaporative cooler type (DEC). The convective heat transfer coefficient correlations are introduced to the fundamental governing equations of this model and the saturation effectiveness of the DEC is calculated. The saline water at different concentrations is used as a feed water to the presented model. The impacts of inlet ambient air conditions and the salinity ratios on the amount of consumed water and the supply air temperature are investigated. The experimental runs are performed on a commercial type DEC and an average operating sprinkled water temperature is found to be 2.5 °C higher than inlet WBT of air. At 200,000 PPM salinity ratio, a reduction in water consumption by about 1.5 L h−1 is reached but with 8.6% increase in the supply air temperature.

An indirect validation of convective heat transfer coefficients (CHTCs) for external building surfaces in an actual urban environment

The convective heat transfer is an important component in the estimation of thermal balance of energy for arrays of buildings immersed in a turbulent boundary layer. This study proposes a novel cost-effective validation approach using targeted field measurements and numerical simulations as an alternative to the wind-tunnel and full-scale field measurement approaches that typically require significant human resources and instrumentation to develop convective heat transfer coefficient (CHTC) for buildings located in the urban environment. This study first introduces new CHTC correlations for regular arrays of cubic buildings. A field measurement is then conducted in an actual urban thermal environment characterized by the plan area density λp = 0.25. Afterwards, this urban thermal environment is numerically simulated using CHTC values for the specified λp and appropriate thermal boundary conditions. The results of the numerical simulations are compared with the measured air temperatures to indirectly validate the CHTCs for external surfaces of buildings. The results show that the difference between the simulated and measured air temperatures is small and typically within 5%. Furthermore, this study created a calibrated building energy simulation model to deploy newly developed CHTCs for building heating energy consumption calculations. Specifically, the energy simulations used both newly developed and commonly used CHTCs to analyze the influence of CHTCs on the heating energy simulation results. A comparison between the simulated heating and actual heating energy consumption shows that the developed CHTCs have a positive influence on the accuracy of the energy simulation results.

The impact of exterior surface convective heat transfer coefficients on the building energy consumption in urban neighborhoods with different plan area densities

In the urban micro-climate environment, the exterior surfaces’ convective heat transfer coefficients (CHTCs) influence simulated building's energy consumption and exterior surface temperatures. This study first investigated the CHTCs at the external windward, leeward, and roof surfaces of a building located in an urban neighborhood with different plan area densities. New CHTC correlations were defined using the plan area density (λp), local velocity (Uloc), and temperature difference (ΔT), and then evaluated in the EnergyPlus simulation program. These new correlations and other commonly used correlations were employed to quantify the impact of CHTCs on the simulation results. The results show that the total cooling energy consumption increased by 4%, and the total heating energy consumption decreased by 1.3% when the plan area density increased from λp=0.04 (almost isolated building) to λp=0.44 (denser cities) due to the convective heat transfer from exterior building surfaces. With variable exterior surface CHTCs, the exterior surface temperature comparisons resulted in maximum temperature differences of 9.1K and 3.6K for the exterior south and north walls, respectively. Overall, this study quantified the impact of exterior surface CHTC correlations on the simulations results that directly impact the urban micro-climate.

Numerical simulation of convective heat transfer coefficients at the external surfaces of building arrays immersed in a turbulent boundary layer

The convective heat transfer is an important component in the total energy balance for arrays of buildings immersed in a turbulent boundary layer. This study examines convective heat transfer coefficients (CHTC) at the external windward, leeward, lateral and top surfaces of buildings. This study uses Large Eddy Simulations (LES) with Smagorinsky–Lily model to predict CHTC and then compares the simulation results with experimental data. In addition, steady RANS including the realizable k–ε and the shear stress transport k–ω turbulence models are also validated with the experimental data. However, both of these RANS models overestimated CHTC values for arrays of buildings in contrast to LES predictions. Regular arrays of cubic buildings are modeled to investigate CHTC in an urban environment, which are arranged according to different plan area densities (λp=0.44, 0.25, 0.16, 0.11, 0.063 and 0.04). This morphological parameter (λp) represents different urban neighborhoods and it is used to characterize different flow regimes in an urban environment. The CHTC distributions are independent of the Reynolds number based on different incoming wind velocities at a height of 10m above the ground (U10) for windward, leeward, lateral and top building surfaces. Furthermore, CHTC distributions for different λp and U10=5m/s are compared with flow characteristics in building arrays. Finally, the CHTC correlations as a function of λp and U10 were obtained, where λp varies from 0.04 to 0.25 and the Reynolds number ranges from 7×105 to 5×106. With the increase of the plan area densities from 0.04 to 0.25, CHTCs increases 15% for the leeward surface and decreases 16% for the lateral surfaces. Consequently, a total energy balance and the energy consumption predictions for a building need to take into account the urban density of the building surroundings.

Experimental investigation of convective heat transfer during night cooling with different ventilation systems and surface emissivities

Night-time ventilation is a promising approach to reduce the energy needed for cooling buildings without reducing thermal comfort. Nevertheless actual building simulation tools have showed their limits in predicting accurately the efficiency of night-time ventilation, mainly due to inappropriate models for convection. In a full-scale test room, the heat transfer was investigated during 12h of discharge by night-time ventilation. A total of 34 experiments have been performed, with different ventilation types (mixing and displacement), air change rates, temperature differences between the inlet air and the room, and floor emissivities. This extensive experimental study enabled a detailed analysis of the convective and radiative flow at the different surfaces of the room. The experimentally derived convective heat transfer coefficients (CHTC) have been compared to existing correlations. For mixing ventilation, existing correlations did not predict accurately the convective heat transfer at the ceiling due to differences in the experimental conditions. But the use of local parameters of the air flow showed interesting results to obtain more adaptive CHTC correlations. For displacement ventilation, the convective heat transfer was well predicted by existing correlations. Nevertheless the change of floor emissivity influenced the CHTC at the surface of interest.

Analysis of convective heat transfer at building façades in street canyons and its influence on the predictions of space cooling demand in buildings

An important part of the world’s energy is used for space cooling of buildings. Minimisation of space cooling, especially in hot urban environments, has great energy-saving potential. An important part of the heat exchange between buildings and the ambient surrounding is due to convective and radiative heat flows. The impact of these heat flows on energy consumption for cooling is much more important in urban areas compared to rural areas. This study aims at quantifying the influence of the urban radiation balance, the urban heat island effect and urban convective heat transfer coefficients (CHTC) on the space cooling demands. CHTC correlations were determined using computational fluid dynamics (CFD) for different geometries including stand-alone buildings and street canyons of different lengths. Buoyancy was accounted for by considering differences between building surface and surrounding air temperature. It was found that the building geometry has a large impact on the CHTC correlations and that the effect of buoyancy cannot be neglected when wind speeds are low. These CHTC correlations were used for Building Energy Simulation (BES) predictions of the space cooling demand. Space cooling demands for a building in a street canyon differ up to a factor of 1.8 depending on the CHTC correlations used. Therefore, for accurate predictions of the space cooling demand, adequate CHTC correlations have to be used adjusted to the actual building configuration.

Convective heat transfer coefficients for exterior building surfaces: Existing correlations and CFD modelling

Convective heat transfer at exterior building surfaces has an impact on the design and performance of building components such as double-skin facades, solar collectors, solar chimneys and ventilated photovoltaic arrays, and also affects the thermal climate and cooling load in urban areas. In this study, an overview is given of existing correlations of the exterior convective heat transfer coefficient (CHTC) with the wind speed, indicating significant differences between these correlations. As an alternative to using existing correlations, the applicability of CFD to obtain forced CHTC correlations is evaluated, by considering a cubic building in an atmospheric boundary layer. Steady Reynolds-averaged Navier–Stokes simulations are performed and, instead of the commonly used wall functions, low-Reynolds number modelling (LRNM) is used to model the boundary-layer region for reasons of improved accuracy. The flow field is found to become quasi independent of the Reynolds number at Reynolds numbers of about 10 5. This allows limiting the wind speed at which the CHTC is evaluated and thus the grid resolution in the near-wall region, which significantly reduces the computational expense. The distribution of the power-law CHTC- U 10 correlation over the windward and leeward surfaces is presented ( U 10 = reference wind speed at 10 m height). It is shown that these correlations can be accurately determined by simulations with relatively low wind speed values, which avoids the use of excessively fine grids for LRNM, and by using only two or three discrete wind speed values, which limits the required number of CFD simulations.

New external convective heat transfer coefficient correlations for isolated low-rise buildings

Building energy analyses are very sensitive to external convective heat transfer coefficients so that some researchers have conducted sensitivity calculations and proved that depending on the choice of those coefficients, energy demands estimation values can vary from 20% to 40%. In this context, computational fluid dynamics calculations have been performed to predict convective heat transfer coefficients at the external surfaces of a simple shape low-rise building. Effects of wind velocity and orientation have been analyzed considering four surface-to-air temperature differences. Results show that the convective heat transfer coefficient value strongly depends on the wind velocity, that the wind direction has a notable effect for vertical walls and for roofs and that the surface-to-air temperature difference has a negligible effect for wind velocity higher than 2 m/s. External convective heat transfer coefficient correlations are provided as a function of the wind free stream velocity and wind-to-surface angle.