Elsevier

Applied Energy

Volume 242, 15 May 2019, Pages 107-120
Applied Energy

Flow and heat transfer characteristics of natural convection in vertical air channels of double-skin solar façades

https://doi.org/10.1016/j.apenergy.2019.03.072Get rights and content

Highlights

  • A numerical model is established to investigate the flow and heat transfer process in vertical channels.

  • The flow and heat transfer characteristics in vertical channels are summarized.

  • The influence factors of the turbulent kinetic energy, velocity and temperature fields are evaluated.

  • The changing tendencies of the induced air flowrate and the temperature rise are investigated.

  • The critical channel widths are suggested for vertical channels with different purposes.

Abstract

Design and construction of internal ventilated air layers have become a popular way to improve the thermal performance of exterior envelopes in modern buildings. These air layers provide multiple benefits to the building envelopes, including improving the thermal insulation property, as well as achieving the effects of fresh air preheating, space heating, natural ventilation, passive cooling, etc. Obviously, the flow and heat transfer condition of the solar driven natural convection in these air layers can significantly influence the performances of these envelopes. This study numerically investigates the flow and heat transfer process, as well as the influence factors of the temperature and velocity fields, the induced air flowrate and the temperature increase in these air layer structures. The results demonstrate that the flow transition, velocity promotion and temperature increase mainly occur in the near-wall regions. For vertical air layers with the height of 2–4 m, the width of 0.1–0.8 m, and the input heat flux of 100–400 W/m2, the air flowrate varies between 0.042 kg/s and 0.255 kg/s, and the range of the temperature rise is 0.66–14.70 °C. For air layers intending to improve ventilation capacity, the channel width should not be bigger than 0.6 m, while for those with the purpose of supplying warm air, the width should be lower than 0.2 m.

Introduction

In buildings, exterior envelopes act as the thermal interfaces between the indoor and outdoor environments, thus can greatly influence the indoor thermal condition and the overall energy consumption of indoor air-conditioning systems [1]. As reported, approximately 20–50% of indoor heating, ventilation and air-conditioning (HVAC) energy consumption is resulted from the heat gain or loss through building envelopes [2]. Actually, the building energy demand is closely linked to exterior envelopes’ thermal performances. If the thermal property of a building’s envelope is poor and unreasonably designed, excessive heat gain and heat loss will occur in these structures, resulting in a significant increase in the energy requirement of indoor air-conditioning systems. Therefore, it is essential to develop high-performance exterior envelopes to guarantee both low energy consumption and high indoor thermal comfort level for modern buildings.

A number of measures can be taken to enhance building envelopes’ thermal performance, such as employing advanced building materials, adding insulation layers, and improving the envelope structure [3]. Currently, the former two measures have been intensively studied by many researchers, and have become commonly applied in modern building construction. For the third measure, in recent years, a commonly used structural design strategy is to add internal ventilated air layers to building envelopes. Nowadays, various types of air layer involved envelope components can be found in existing buildings, such as Trombe walls [4], [5], solar wall/chimney [6], double-skin façades [7], [8], and ventilated PV façades [9], [10]. Air layers used in these components provide multiple benefits for buildings. When the air layer operates in a closed mode, it performs as an extra insulation layer for exterior envelopes, thus improving the thermal insulation property. When the air layer works in an open-ended mode, ventilation channels are generally formed in between the double-layer envelope structures. With the promotion of the buoyancy force resulted from the solar radiation, the ambient air is induced into the channel from the bottom and discharged at the top. The upper and lower vents of the channel could be connected to either the outdoor or the indoor environment, to produce various air circulation patterns. In accordance with the air circulation pattern, multiple functions could be achieved, such as fresh air preheating and supplying, space heating, natural ventilating, and passive cooling [10], as illustrated in Fig. 1.

There are numerous literatures focusing on the performances of the air layer envelopes in buildings. Through an experimental study, Chen [11] concluded the effect of using a Trombe wall on reducing the energy demand of an office room. The results indicated that energy saving was 30% when using the Trombe wall. Sun et al. [12] tested and simulated the operating conditions of a PV-Trombe wall with different design configurations. The results verified that employing a window on the south façade was beneficial for indoor warming, but it reduced the thermal efficiency of the wall system by nearly 27%. Hu et al. [13] reported the comparative assessment results of different PV-Trombe walls focusing on the heating/cooling load reduction effect and electricity generation. Hirunlabh et al. [14] investigated the overall performance of a solar chimney made mainly from metallic materials. During their test, when the chimney area was 2 m2, the induced flowrate achieved 0.01–0.02 kg/s, and the indoor heat gain was significantly reduced. Peng et al. [15] evaluated the overall performance of ventilated PV façade under different ventilation strategies. The tested results showed that the ventilated mode performed better for improving the electricity output and reducing the solar heat gain. Yang et al. [16] concluded that the cooling load reduction potential of the ventilated PV wall was 33–50%. Gagliano et al. [17] studied the thermo-fluid dynamic behavior of an opaque ventilated façade. The numerical results indicated that this façade offered an energy saving potential of 47–51% depending on climate conditions. Harris [18] comparatively studied the performance of a solar chimney with different inclination angle and surface emissivity through numerical simulations. The results showed that the optimal inclination angle of the solar chimney was 67.5°, with which the energy efficiency was 11% higher than the vertical configuration. Li [19] tested the effect of employing phase change material (PCM) on a solar chimney’s performance. The experiment showed that the PCM-combined solar chimney had an acceptable performance, but the performance was greatly influenced by the property of the PCM. Zamora and Kaiser [20] investigated the performance of a solar chimney driven by the combination of the wind and the buoyancy. A correlation of the non-dimensional mass flowrate was established for this type of solar chimney. Arce et al. [21] experimentally tested the thermal and ventilation behavior of a solar chimney. The results presented that the air temperature promotion through the chimney could reach up to 7.0 °C, and the induced air flowrate varied in the region of 50–374 m3/h. Yu et al. [22], [23] evaluated the overall performance of a thermal catalytic Trombe wall. The results verified that this wall system enjoyed an overall energy saving potential o 97.4 kWh/m2, including 64.3 kWh/m2 for space heating and 33.1 kWh/m2 for formaldehyde degradation.

As verified by the aforementioned literatures, using air layers in exterior building envelopes greatly improves the thermal performance, and provides an energy saving potential in indoor air-conditioning. It is noteworthy that the thermal performances and the overall energy efficiencies of these envelopes are closely related to the air flowrate and the air temperature rise through the air channels. Moreover, these two parameters are mainly determined by the flow and heat transfer condition within the internal confined space of the air channels. Therefore, it is essential to identify the operating characteristics of these air channels, as well as to investigate the influence factors of the induced air flowrate and the air temperature rise. Previous research indicated that the airflow and heat transfer processes of the natural convection in the vertical air channels might be laminar or turbulent depending on the channel’s thermal condition and geometrical size. As reported, for the vertical air channels in building envelopes, the buoyancy driven natural convection always transforms from the laminar state to the turbulent state as the heat transfer progresses [24]. Although a large number of literatures can be found focusing on the performance of air layer envelopes. However, only a few investigations were reported on the detailed flow and heat transfer process in the vertical air channels.

Some relevant experimental test results were reported. Miyamoto et al. [25] performed a famous experimental study on the heat flux driving natural convection in such vertical air channels, and concluded a correlation equation for the heat transfer for this channel. Fraser et al. [26] reported their test results on the temperature profiles of the air in these vertical channels. La Pica et al. [27] reported their results of the correlations of average Nusselt number and Reynolds number in vertical air channels. Yilmaz and Fraser [24] also reported their tested and simulated results on the profiles of the air velocity and air temperature at different heights of the channel. These investigations provided limited results for the natural convection in the vertical air channels, due to the limitations of the instrumentation and test point arrangement. As a result, numerical methods were employed for detailed study on the operating conditions of these air channels. Fedorov and Viskanta [28] numerically simulated the temperature and velocity distributions within a vertical channel symmetrically heated by constant heat flux of 80–208 W/m2. Employing the Low Reynolds number k–ε model, the distributions of the Nusselt number and the local heat flux were reported. Refs. [29], [30] reported the simulation results on the convection in the vertical air channels with different thermo-physical conditions and geometrical parameters. Ben-Mansour et al. [31] compared the accuracy of the available turbulent models in predicting the natural convection behavior in vertical air channels. The results presented that the predicted temperature results based on the low Reynolds number k–ε model agreed best with the test results reported in [25], but the differences between the simulation results of standard, renormalization group (RNG), realizable and low Reynolds number k–ε models were very small and could be ignored. Alzwayi and Paul [32] simulated the flow transition phenomenon in the natural convection in vertical air channels. They concluded that the differences between the simulated results from different turbulent models were very small. Gan [33] also compared the suitability of several turbulent models in predicting the flow and heat transfer pattern in vertical air channels. The results showed that the employed models gave similar results for the temperature and velocity field, but the standard k–ε model was more efficient and less time-consuming in the simulation.

Seen from the above analysis, many researchers investigated the natural convection in the vertical air channels of building envelopes, focusing on the air temperature and velocity distributions, and many scholars carried out vast investigations on the performances of different types of air channel involved building envelopes. However, none of these researches aimed at finding the general characteristics of the solar driven natural convection in these air channels. The detailed flow and heat transfer process and its influence factors, the induced air flowrate and the temperature rise through the air channel involved envelopes, have not yet been deeply discussed. Moreover, the relationship between the flow, heat transfer condition and the performances of these envelope components are still not clearly identified. Therefore, there is still a need to perform in-depth study on the flow and heat transfer characteristics in the vertical channels of building envelopes.

This paper aims to present a detailed numerical simulation on the heat flux driving natural convection in the vertical air channels, to investigate the distributions of the air temperature and air velocity. Furthermore, the influences of geometrical sizes and input heat flux on the flowrate and temperature rise through vertical air channels are also necessary to be discussed. The research outcomes can provide necessary technical support for the design and construction of the vertical air channels in building envelopes.

Section snippets

Physical and mathematical models

All the air channel involved building envelopes have the similar structure. Firstly, these envelopes usually have two structural layers, and an air channel is reserved in between the two structural layers. Secondly, solar absorbing materials, such as solar collector, PV module, absorptive glass, and metallic plate, are usually employed in one of the structural layer to absorb as much solar energy as possible. This work focuses on the internal vertical air layer of double-skin solar façades,

Flow and heat transfer characteristics of the heat flux driving natural convection in the vertical channel

In this study, numerical simulations were performed on the vertical air channels with different geometrical sizes and heat fluxes. The channel width ranges from 0.1 m to 0.8 m and the height changes between 2 m and 4 m. The input heat fluxes on the right wall ranges from 100 W/m2 to 400 W/m2; accordingly, as mentioned above, the input heat fluxes on the left wall ranges from 7.7 W/m2 to 30.8 W/m2. The inlet temperature is set to be 20 °C.

Influences of geometry parameters and input heat flux on the turbulent kinetic energy, velocity and temperature fields

In the natural convection of a vertical channel, the flow and temperature distributions are influenced by many factors, such as the channel size and the heat flux boundary. In this section, the effect of these factors will be assessed.

Influence factors of temperature rise and induced flowrate

Due to the purpose of using air channels in building envelopes, the airflow rate and air temperature rise across the vertical channel are worthy of investigating. The simulation results show that for vertical channels with the height of 2–4 m, the width of 0.1–0.8 m, and the input heat flux of 100–400 W/m2, the air flowrate varies between 0.042 kg/s and 0.255 kg/s, and range of the temperature rise is 0.66–14.70 °C.

Fig. 20 illustrates the changing tendencies of the flowrate and temperature rise

Conclusions

This study investigates the flow and heat transfer characteristics of natural convection in vertical air channels of building envelopes. Based on the above results and discussions, the following conclusions have been drawn:

For the heat flux driving natural convection in vertical air channels, the values of the turbulent kinetic energy, the velocity and the temperature in the near-wall regions are much higher than in the central region of the vertical channel. Therefore, the flow transition,

Acknowledgements

This research was supported by The Hong Kong Polytechnic University Postdoctoral Fellowships Scheme (G-YW2E).

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