Turbulent mixed convection in an enclosure with different inlet and outlet configurations

Given the large impact of the building sector on the final energy demand, special interest lays in passive cooling techniques, such as night ventilation. Unfortunately, a lack of understanding concerning the coupling between the ventilation air and the thermal mass by night stands its widespread application in the way. Therefore, the authors of this paper investigate by computational fluid dynamics turbulent mixed convection cooling in a rectangular enclosure – resembling a night ventilated landscape office. Based on the 2D Annex 20 test case, four different orientations of the inlet and outlet are considered while varying the Archimedes number – ranging from forced to mixed convection. Also the location of thermally massive elements is varied. Locating the inlet and outlet at the top of the room induces the highest convective heat transfer. Meanwhile, locating the thermal mass at the floor has more potential than at the ceiling

Convective Heat Transfer coefficients in mechanical night ventilation: a sensitivity analysis

Since the Energy Performance for Buildings Directive (EPBD) was accepted and implemented over the course of the last years, buildings are audited energetically to receive the necessary construction licenses. This augmented the already high attention to research on innovative (passive) energy-saving system concepts even further. Previous research suggests that, although the effect of commissioning can be significant, specific fan power is the most important factor influencing the energetic viability of mechanically driven night ventilation as an active cooling replacement. This parameter should thus be the central point of focus during the design process. In this paper, we present an analysis of the effect of detailed convective heat transfer modeling on the predicted performance, in order to determine the level of detail needed to assess feasibility of this kind of system in early design phases. Results indicate that the effect amounts to 20-50% of the predicted performance and therefore cannot be neglected. It is within the range of effect of the dominant parameter, specific fan power. In light of these results, it is suggested that detailed convective heat transfer coefficient modeling is taken into account whenever forced convection due to large volume flow is introduced

Sensitivity of night cooling performance to room/system design: surrogate models based on CFD

Night cooling, especially in offices, attracts growing interest. Unfortunately, building designers face considerable problems with the case-specific convective heat transfer by night. The BES programs they use actually need extra input, from either costly experiments or CFD simulations. Alternatively, up-front research on how to engineer best a generic night cooled office – as in this work – can thrust the application of night cooling. A fully automated configuration of data sampling, geometry/grid generation, CFD solving and surrogate modelling, generates several surrogate models. These models relate the convective heat flow in a night cooled landscape office to the ventilation concept, mass distribution, geometry and driving force for convective heat transfer. The results indicate that cases with a thermally massive floor have the highest night cooling performance

Sensitivity analysis of predicted convective heat transfer at internal building surfaces to diffuser modelling in CFD

As a cost-effective alternative to experiments, computational fluid dynamics (CFD) can provide new insight in airflow patterns and the related convective heat transfer (CHT). However, together with the governing equations, the description of the boundary conditions determines for a greater part the reliability and the accuracy of CFD simulations. In this study the sensitivity of the predicted CHT to diffuser modelling is studied. Numerical simulations of a modified version of test case E.2 of the IEA Annex 20-project are performed. The results show a strong influence of the applied diffuser model on the predicted CHT

On modelling moisture buffering when evaluating humidity controlled HVAC systems

As most building energy simulation programs focus on the thermal response of the building, the relative humidity of the indoor air is often calculated in a simplified way. One of the main shortcomings is the isothermal calculation, which may have a strong influence the predicted relative humidity. In this paper the use of a simplified effective moisture penetration depth (EMPD) model is compared with a coupled TRNSYS-HAM-model. First, an estimation of the load for humidification and dehumidification is made. Results showed that the EMPD-model underestimates the humidification load because the model disregards non-isothermal effects. Secondly, calculations showed that the indoor and surface relative humidity of an office room with a gypsum cooled ceiling are overestimated using the EMPDmodel. Furthermore, due to not including nonisothermal effects the peak load for dehumidifying the ventilation air may be underestimated using an EMPD-model

Sensitivity analysis of thermal predictions to the modeling of direct solar radiation entering a zone

Because of high computational costs of computational fluid dynamics multi-zone energy simulation is currently appraised. Yet, the use of empirical correlations to predict interior convective heat transfer (CHT) limits the reliability of building comfort and energy analysis. As most of these convection algorithms depend, partially, on the temperature difference between the concerned surface and the air, the influence of the modeling of incoming direct solar radiation is studied. Simulations of summer comfort in a night cooled office room in Belgium are carried out in TRNSYS using different convection algorithms and four methods to model the distribution of solar radiation. This work demonstrates that the influence of the modeling of incoming solar radiation on the predicted thermal comfort and energy demand is inferior to the choice of the CHT correlations. Therefore, before putting considerable effort in the modeling of direct solar radiation, an accurate approach of the CHT is regarded necessary

Convective heat transfer modelling in offices with night cooling

Night ventilation to cool buildings attracts growing interest. For, it can improve the summer comfort and can lower the cooling need. However, the extent to which building designers succeed in finding an optimal night cooling design depends strongly on the simulation tool they use. Today, stand-alone building energy simulation (BES) programs are quite popular, but the way they model the convective heat transfer raises questions. They model the complex heat transfer in the boundary layer and the surrounding field by a convective heat transfer coefficient which relies primarily on case-specific experimental data. Therefore, this thesis evaluated whether this modelling approach suffices to accurately predict the night cooling performance and further investigated the impact of the room/system design on the convective heat transfer during night cooling. The work began with a literature review which highlighted the limited applicability of the various convection correlations. It appeared that the researchers involved successively developed correlations for distinct cases which had not been studied yet and that a number of people already suggested to categorize all situations into a discrete number of regimes to which specific correlations apply. The subsequent BES-based sensitivity analysis in this thesis indicated that such a pragmatic approach is indeed no luxury. However, as shown in the experimental study in the PASLINK cell, part of this work, it does not enable to investigate the influence of a parameter (value) other than the ones considered in the experimental setup. So, it is necessary to further investigate how room/system design parameters affect the convective heat transfer and eventually refine the BES approach. The second part of the thesis dealt with the way to do this and presented a pilot study on a night cooled landscape office. Surrogate modelling in conjunction with computational fluid dynamics (CFD) would be a valuable supplement to experiments; on condition that CFD users know how to address the inherent error sources. A fully-automated framework of data sampling, geometry/grid generation, CFD solving and surrogate modelling was set up and then deployed to investigate how the convective heat flux in a night cooled landscape office relates to the room/system design. The resulting surrogate models provided rough-hewn insights and, more importantly, the framework can be reused to derive more globally accurate surrogate models which can be coupled with BES

Experimental investigation of the impact of room/system design on mixed convection heat transfer

Night cooling attracts growing interest. However, architects and engineers still hesitate to apply night cooling because of the important but hard-to-predict convective heat transfer by night. Obviously, this heat transfer mechanism depends on the driving force, fluid motion and heat transfer surface and, thus, on the room and system design. Unfortunately, studies addressing this are scarce. In response, underlying experimental effort intends to instigate global parametric analyses of night cooling at room level. To this end, this study, held in a PASLINK cell, investigates how the ventilative cooling rate, thermal mass and the supply/exhaust configuration affect the convective heat transfer. The analysis is based on airflow data, such as temperature and velocity, and the related convective heat flux. The results indicate the need for an integrated room/system design. After all, the position of the supply relative to thermally massive elements predominates the night cooling performance