Copyright 2003, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Reprinted by permission from the International Journal of HVAC&R Research, Vol. 9, No. 3, July 2003. This article may not be copied nor distributed in either paper or digital form without ASHRAE's permission.
VOL. 9, NO. 3
Impact of Mixed Convection on Ceiling Radiant Cooling Panel Capacity Jae-Weon Jeong
Student Member ASHRAE
Stanley A. Mumma, Ph.D., P.E.
The main thrust of the research described in this paper was to develop a simplified method of accurately estimating the impact of mixed convection on the cooling capacity of a ceiling radiant panel in mechanically ventilated spaces. The simplified correlation for mixed convection heat transfer was derived from established mixed and natural convection correlations. It was found that the total capacity of ceiling radiant cooling panels can be enhanced in mixed convection situations by 5% to 35% under normal operating temperatures.
Currently, most ceiling radiant cooling panel (CRCP) performance estimates are based on natural convection only. This is reflected in ASHRAE (2000) literature, where the analysis is based upon the natural convection heat transfer work of Min et al. (1956), and the European CRCP capacity rating standard, DIN 4715 (1997), which uses natural convection as the test condition. However, Kochendörfer (1996) found that in real buildings, cooling outputs of CRCPs are significantly higher (25%) than measured panel capacities tested in the laboratory under DIN 4715 conditions. In real buildings, mechanical ventilation systems are usually used, and the walls are not adiabatic. If the higher performance of CRCPs is ignored in the design phase, unnecessary panel area is specified and the cost of the panels is excessive.
The two major sources of reliable building-related natural convection heat transfer coefficients are Awbi and Hatton (1999) and Min et al. (1956). The natural convection coefficient proposed by Min et al. is as follows: h c = 2.13 ⋅ ( T a – T pm )
A number of works referenced in the literature that deal with mixed convection include Chen et al. (1989), Fisher and Pedersen (1997), and Awbi and Hatton (2000). Awbi and Hatton proposed mixed convection heat transfer coefficients for heated room surfaces partially covered by air jet. The correlation for a heated floor or cooled ceiling is as follows: h c = ( h cn + h cf )
3.2 3.2 1 ⁄ 3.2
Jae-Weon Jeong is a doctoral student and Stanley A. Mumma is a professor in the Department of Architectural Engineering, The Pennsylvania State University, University Park, Pa.
2003 ASHRAE. This document is not to be distributed without written permission from ASHRAE.
Table 1. Coefficients for Correction Function
a1 0.12933333 a6 -0.049091666 a2 1.294888889 a7 0.00417 a3 0.051308333 a8 0.001202777 a4 -0.29422 a9 -0.000111666 a5 0.016286666
0.308 2.175h cn = -------------- ( T a – T pm ) 0.076 De
h cf = 4.25 ⋅ W
SIMPLIFIED MIXED CONVECTION COEFFICIENT
In this research, the mixed convection heat transfer coefficient developed by Awbi and Hatton (2000) was extensively analyzed to derive a simplified mixed convection coefficient that can be more easily used in the design stage of a CRCP system. Awbi and Hatton’s mixed convection correlation for a cooled ceiling (Equation 2) is a function of the characteristic diameter of a space (De), space-to-panel temperature difference (∆T), diffuser width (W), and diffuser discharge air velocity (V). Forced convection effects, the difference between natural convection coefficients and mixed convection coefficients, for various ∆T and V values were calculated using Equations 1 and 2, respectively. Equation 3a returns the forced convection effect in W/m2⋅K,...
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