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Convective Heat Transfer in Rooms with Ceiling Slot Diffusers (RP-1416)

Kate Goldstein Atila Novoselac, PhDStudent Member ASHRAE Associate Member ASHRAE

Received February 25, 2010; accepted May 14, 2010

This paper is based on findings resulting from ASHRAE Research Project RP-1416.

Convection at interior surfaces significantly affects the overall heat transfer in office buildingswith large glazing areas. While a large number of these office buildings utilize ceiling slot dif-fusers at the glazed building perimeter, convection correlations specific to these diffusers havenot yet been investigated. This paper describes convection correlations developed for ceilingslot diffusers and examines the effect of temperature, window geometry, and diffuser jet momen-tum on these correlations. The paper also examines the effect of venetian blinds on the overallcorrelations. The convective heat transfer coefficients are measured in both heating and coolingconditions. Overall, this paper represents the outcome of over 100 individual full-scale experi-ments. The results show that forced convection is dominant for a wide range of airflow rates.Correlations are developed as a function of the specific volumetric flow rate of supply air. Thetemperature of the supply air is also utilized as the reference temperature in convection calcula-tions. The correlations are found to rely primarily on window position and the presence ofblinds. The correlations are independent of the temperature difference between surface and sup-ply, diffuser position, and diffuser jet momentum. The paper also quantifies the differencebetween the convection correlations developed for heated and cooled environments. Finally, uti-lizing energy modeling to quantify the difference between the new correlations and those previ-ously used, it is found that former convection coefficients underestimate the energy demand. Inthe modeled building, utilizing the newly developed correlations for forced convection increasedthe peak cooling load by 24% and the overall energy consumption by 4% over the baseline thathad natural convection with constant coefficients at window surfaces.

INTRODUCTIONConvective heat transfer at the interior surfaces of buildings accounts for a great proportion of

the overall thermal load (Awbi 1998; Beausoleil-Morrison and Strachan 1999). Modeling pro-grams rely on convective heat transfer correlations to quantify the magnitude of convection ateach building surface. The correlation values vary widely among different programs and oftenpresume natural convection to be the dominant mode of convective heat transfer. This can seri-ously underestimate the convective heat loss in mechanically ventilated buildings (Awbi andHatton 2000). An incorrect assumption in the modeling of convection, such as inaccurate con-vection correlations, can translate into a large error in the overall estimation of total heat transferwithin the building. In a study examining the significance of varied convective heat transfercoefficients, Lomas (1996) found that the cooling load could vary as much as 27%. Consideringthe building energy consumption prediction, the degree of sensitivity of energy simulation to

Kate Goldstein is a masters student and Atila Novoselac is an assistant professor in the Department of Civil, Architec-tural, and Environmental Engineering at the University of Texas at Austin, Austin, TX.

2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC&R Research (Vol. 16, Issue 5, September 2010). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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convection calculation models is approximately 8% (Beausoleil-Morrison and Strachan 1999;Alamdari et al. 1984; Clarke 1991; Lomas 1996). Convection plays an important role in theaccuracy of the overall heat transfer calculation. A good overall heat transfer model will link theaccuracy of radiation and conduction models to the accuracy of the convection model. In energymodeling tools, indoor radiative heat transfer is usually described by precise radiation equations.The inaccuracy in radiation models comes from the neglect of participating objects within theroom such as people, computers, and lights. Conduction is limited by the accuracy ofone-dimensional homogeneous models used to represent multidimensional heat transfer occur-ring in nonhomogenous materials or building corners. Convection on the other hand is limitedby the accuracy of the analytical or semiempirical correlation used for a characteristic spacegeometry and airflow distribution. This is the accuracy the new convection correlations seek toaddress.

There are significant differences in convective heat transfer not only between mechanicallyand naturally ventilated rooms, but also among different types of air distribution and heating andcooling systems such as: displacement ventilation, mixing ventilation with various ceiling andside wall diffusers, floor registers, radiant floor heating, and cooled ceiling panels (Novoselac etal. 2006a). Among these systems are ceiling slot diffusers, one of the most common airflow dis-tribution systems in modern commercial buildings with large glass facades. With these systems,cold (or hot) air is supplied in the perimeter zones of buildings along the window surfaces to dis-sipate heat (or cover heat losses) at the source locations (Awbi 2003).

Ceiling slot diffusers in perimeter building zones are unique in their proximity to the window.This causes the vertical air supply jet to attach itself to adjacent surfaces. This phenomenon istermed the Coanda effect, and it transports the jet momentum along the window and wall sur-face. In addition to this forced convection, large temperature differences between supply air andwindow surfaces cause buoyancy. Depending on the direction of the surface heat flux (positivefor summer or negative for winter), the surface-air temperature difference can generate a forcethat acts in the same or the opposite direction to the ceiling supply jet. Consequently, heat trans-fer at the window surfaces is potentially affected by both forced and natural convection. Whileat high volume flow rates one would expect the correlations to be dominated by forced convec-tion, at lower flow rates, the large air temperature stratification could cause the effects of buoy-ancy to dominate. As mentioned before, the most commonly used convection correlations atwindow surfaces are based on the assumption that natural convection is the dominant mode ofconvective heat transfer. However, without knowledge of which effect dominates, it is difficultto know whether currently used models underestimate or overestimate convective heat transferin commercial buildings with ceiling slot diffusers.

While specific correlations have been developed and utilized in energy analysis software fordecades, there is currently a gap in the literature concerning forced air cooling and heating sys-tems, including ceiling slot diffusers located near the window (Beausoleil-Morrison and Stra-chan 1999). As mentioned above, this is particularly important because this supply airconfiguration represents a large proportion of office ventilation systems. Furthermore, whilenumerous studies have been conducted to investigate heat transfer correlations in mixed convec-tive regimes (Chen et al. 1989; Fisher 1995; Pavlovic 1991; Novoselac 2005), none of thesestudies focused on ceiling slot diffusers near the window surface, nor on the development of cor-relations specifically pertaining to the window.

Typical office buildings utilize not only ceiling slot diffusers close to the window, but alsoblinds which control the daylighting within the room and reduce the penetration of solar heatthrough the window. While the primary purpose of the blinds is to reduce heat gains from short-wave (solar) radiation, they also influence convective heat transfer as well as heat transfer in aroom by long wave radiation. In addition, blinds affect the competition between natural and

2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC&R Research (Vol. 16, Issue 5, September 2010). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission.

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forced convection and affect the solar heat gain and overall U-value of the window (Collins2005). Oosthuizen (2008) and Collins et al. (2008) investigated the effect of blinds geometry onconvective heat transfer for different configurations of blinds and windows. Wright et al. (2009)developed an empirical model that predicts total (convective and radiative) heat transfer fromthe interior of a window with a venetian blind. However, the model is primarily applicable fornatural convection and does not include the effect of forced convection, which is particularlystrong in the presence of ceiling slot diffusers.

The objective of the study described in this paper is to determine heat transfer correlationsfor internal surfaces within office buildings that utilize ceiling slot diffusers in perimeter build-ing zones. Particular attention is paid to the large glazing areas typical in office buildings, andthe effect of venetian blinds on the convection at window surfaces. Part of the challenge inmodeling convection and developing accurate correlations for interior room surfaces lies indesigning ex