Influence of the Radiative exchange in the Heat Transfer. of a Cubic Cavity with Semitransparent Wall using. Room temperature Conditions

J. J. Flores’ and G. Alvarez +

CENIDET, Mechanical Engineering Department. Prolong. Av. Palmira s/n. Col. Palmira.
Cuernavaca, 62490, Morelos, Mexico. Tel./Fax: +52 777 312-7613. Emails:
jasson@cenidet. edu. mx, gabv@cenidet. edu. mx.

+CIEMAT. Renewable Energy Department. Av. Complutense No. 22. Madrid, Spain.

28040. Tel/Fax: 91 346 6344.

Abstract

This paper presents the influence of the radiative exchange of a three dimensional cavity with a semitransparent wall with solar control coating, considering that the temperature distribution of the test glass is function of the thermal interaction between the interior and the exterior of the cavity. The theoretical heat transfer model was compared with experimental measurements. The amount of the radiative and convective terms are quantified and their results are that the radiative term influences the heat transfer as much as the convective term in 12.4%. Also the present results of theoretical heat transfer model were compared with theoretical and experimental results reported in the literature. These results show that the maximum difference was of 5.58% be total Nusselt number and for the individual radiative and convective Nusselt numbers the difference increases.

1. Introduction

The evaluation of solar heat gains or losses through windows implies to calculate the amount of solar energy that is able to cross through the glass towards the interior of a room with respect to the one that strikes on the outside of the glass. From the solar energy that goes into the glass, some of that energy is transmitted, another is reflected and the rest is absorbed. The absorbed solar energy in the glass is transported by conduction towards the interior of the room and is transferred by convection and radiation from both sides of the glass to the interior and exterior air. Therefore, to quantify the amount of solar energy transported to the interior, it is necessary to evaluate the direct solar energy transmitted and the fraction of energy that is transferred in the form of thermal heat by convection and radiation.

To make the evaluation of the fraction of solar energy that is transferred by the glass towards the air in the room, some thermal parameters have been defined, such as: the Shading Coefficient (SC) [AsHRAE, 1997], the Solar Heat Gain Coefficient (SHGC) [ASHRAE, 2001] and the Solar Rejection Factor (SRF) [Alvarez, 1994]. To calculate these parameters, it is necessary to evaluate the overall heat loss coefficient of the glazing that depends on the convective and radiative heat transfer coefficients. The evaluation of these heat transfer coefficients can be carried out by idealized experimental or theoretical models. Hollans et al. in 1976, ElSherbiny et al. in 1982, Pepper and Hollands in 2002, ISO-9050 in 2001, ASHRAE in 2001 among other authors determined several theoretical
correlations for those heat transfer coefficients. Janssen and Henkes in 1995, Leong et al. in 1998 and Leong et al. in 1999 among other authors determined several experimental correlations. Most reported theoretical studies of the transport of heat in rooms consider modeling a room as a closed cavity heated differentially on the vertical walls, considering just heat transfer by natural convection. Very few studies in cavities consider the three modes of heat transfer: convection, conduction and radiation; and even lesser studies consider semi-transparent walls or windows. Among the studies that consider semitransparent walls, most of them are in two dimensions and only Alvarez in 1994 and 2000 consider a solar control coating on the glass.

In this paper we present the influence of the radiative heat transfer of a three dimensional cavity with a semitransparent wall with solar control coating, considering that the temperature distribution of the test glass is function of the thermal interaction between the interior and the exterior of the cavity.

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *