Natural Ventilation of Wall Air Cavity for Solar Heat Gain Reduction : Part 5-Heat Transfer Coefficient in Cavity and Unsteady State Heat Transfer Simulation

KADOYA Terunori, HOMMA Hiroshi

doi:10.18948/shase.14.41_61

Natural ventilation of an air cavity in a building wall has long been practiced as measures for internal condensation prevention in a cold district, and for radiative heat dissipation in a hot district. But this technic has been developed empirically, and its scientific base has not yet been known sufficiently. A building wall is exposed to a daily change of weather, especially of solar radiation. So the thermal response of a naturally ventilated cavity wall must be examined under an unsteady heat transfer condition. For this purpose, a simple method is required to simulate the thermal effect of natural ventilation in a cavity. Natural ventilation in a wall cavity changes from laminar flow, when solar radiation is scarce, to turbulent flow, when a wall exterior recieves strong solar radiation. There exists abundant experimental and theoretical research results on the heat transfer of a heated internal surface of a duct or a pipe, which is ventilated naturally. But most of them belong to either one of the regions of laminar and turbulent flow. Natural convection heat transfer is not yet thoroughly studied for the transition region between them, where the natural ventilation of a cavity performs for a long duration. So the mass and heat transfer in wall cavities was examined with full scale thermal models of wall cavities. The thickness of the cavities, the contraction at the entrances and exits of the air passages and the heat flux of the walls were changed in ranges. An experimental equation was induced from the results of the heat transfer experiment. The applicable region of this equations is limited in the area, where natural ventilation in a wall cavity may experience. A numerical analysis computer program was made to simulate the response of a naturally ventilated cavity wall under a daily change of outdoor and indoor conditions. This program consists of two parts. In one of the parts, the horizontal heat transfer across the cavity and the vertical air flow and its heat transportation effect is processed. The proposed equation for the heat transfer in a cavity is adopted in this process. In the other part, the heat transfer through the solid part of the wall is treated as transient and two dimensional heat flow. The simulation results of this computer program was examined by comparing it with the experimental results of the two typical wall models, which were exposed to the weather of a typical Japanese country side. This comparison indicated that the numerical analysis program simulated the temperature variation in the walls, and the heat flux on the external and internal surfaces of the wall main bodies reasonablly well. This numerical analysis program may be used effectively for performance examination of natural cooling of cavity ventilated walls, which may have various configurations and various thermal characteristics, and may be exposed to various climates.

Natural Ventilation of Wall Air Cavity for Solar Heat Gain Reduction : Part 5-Heat Transfer Coefficient in Cavity and Unsteady State Heat Transfer Simulation

Bibliographic Information

Search this article

Description

Journal

Keywords

Details 詳細情報について

Export

Report a problem