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Interior Convective Heat Transfer in Buildings with Large Ventilative Flow Rates, ASHRAE Transactions

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Interior Convective Heat Transfer in Buildings with Large Ventilative Flow Rates, ASHRAE Transactions This paper has been downloaded from the Building and Environmental Thermal Systems Research Group at Oklahoma State University (www.hvac.okstate.edu) The correct citation for the paper is: Spitler, J., C. Pedersen, D. Fisher. 1991. Interior Convective Heat Transfer in Buildings with Large Ventilative Flow Rates, ASHRAE Transactions. 97(1): 505-515. Reprinted by permission from ASHRAE Transactions (Vol. #97, Part 1, pp. 505-515). © 1991 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. NY-91-5-2 (RP-529) INTERIOR CONVECTIVE HEAT TRANSFER IN BUILDINGS WiTH LARGE VENTiLATIVE FLOW RATES J.D. Spi.tler, Ph.D., P.E. C.O. Pedersen, Ph.D. D.E. Fisher Associate MemberASHRAE Fellow ASHRAE Associate MemberASHRAE ABSTRACT transfer is extremely important; film coefficients should be considerably higher than those normally used. This paper presents the results obtained from a new This paper describes the results of an experimental experimentalfacility designedto investigate convective heat investigation of convective heat transfer in enclosures under transfer. It is described in a companionpaper. A large conditions that represent a wide range of ventilative condi- number of experiments were performed with varying inlet tions for buildings. The lowest volumetric flow rate locations and sizes, inlet temperatures, and flow rates. The examined, 15 ach, is at the high end of what might be rootn outlet temperature was identified as the most suitable considered typical for an occupied office building. Flow reference temperature for the calculation of film coeffi- rates above 15 ach might be encountered when certain cients. Film coefficients were successfully correlated to the ventilative cooling strategies are implemented.Under these jet momentumnumber, J. The correlations form the basis of conditions, the jet and surrounding flow are generally a new convective heat transfer model. This model has been turbulent. The experimentalenclosure used in this investiga- tested in the Building Loads Analysis and System Thermody- tion has approximateinterior dimensionsof 15 ft by 9 ft by namics (BLAST) program. The new model enabled BLAST 9 ft (4.6 m by 2.7 m by 2.7 m) and is considered full scale. to accurately predict the experimental results. The walls, floor, and ceiling are covered by individually ht addition, an office building with a night purging controlled heated panels that were maintained at 86°F strategy was simulated, using both the current model, based (30°C) for all experiments. Heat fluxes from each panel can on natural convection, amt the new model. The two models be determined using an energy balance. The room is well predicted significantly different ventilative cooling rates. instrumented, with more than 100 surface thermocouples, additional thermocouplesfor resolving all energy flows, and INTRODUCTION airspeed sensors and thermocouples that measure air speed and air temperature at 896 points. The ventilation system is An important componentof building energy analysis is capable of providing more than 100 ach, but it can be the prediction of interior convective heat transfer. The throttled downto provide as few as 2 ach. The experimental model currently used is knownas the well-stirred model facility is described in detail in a companionpaper (Spitler and assumes that the zone air has uniform temperature and et al. 1991). that a convective heat transfer coefficient, or film coeffi- The primary objective of this work is to create an cient, can be selected to predict the convective heat trans- experimental background for the development of improved fer. The coefficients given by the 1.989 ASHRAEHand- interior convective heat transfer models for use in building bookwFundamentals and commonly used by building simulation programs. These models will primarily be in the energy analysis programs appear to be based on research form of correlations for film coefficients. With this type of performed in the 1930s (Wilkes and Peterson 1938). model, the important questions to be answered are: Furthermore, this research was based on experimental measurementof heat transfer from fiat plates in a free 1. Whatis an appropriate reference temperature? environment under conditions of natural convection. The 2. Whatparameters influence the film coefficient for applicability of the well-stirred model whencoupled with the conditions of interest? .... these film coefficients has been questioned by numerous researchers (Gadgil 1980; van der Kooi and Forth 1985; Hourly building energy analysis programs simulate Lebrun and Ngendakumana1987; Chen and van der Kooi 8,760 hours for an annual simulation. This leads to some a988). constraints on the complexity of a model that can be The applicability of the natural-convection-based film effectively utilized. Furthermore, since there are many coefficients is particularly questionable under ventilative unpredictable simulation parameters (such as occupants) that cooling conditions, such as night purging. Night purging influence the roomair flow and hence, the convective heat involves ventilating the building at night to precool the transfer, there is a point beyondwhich increased accuracy building structure. Night purging may also involve very maybe a waste of effort. high flow rates, perhaps as high as 100 air changes per hour (ach). The coupling betweenthe building mass and the EXPERIMENTAL RESULTS ventilation air described by the convective heat transfer model is the dominant effect in the simulation of night A total of 44 experimental tests were performed. This purging. In this case, accurate modelingof convective heat included one set of 30 parametric tests, in whichvolumetric Jeffrey D. Spider is an Assistant Professor, Schoolof Mechanicaland AerospaceEngineering, Oklahoma State University, Stillwater; Curtis O. Pedersenis a Professor and Daniel E. Fisher is a ResearchEngineer, Departmentof Mechanicaland Industrial Engineering, Universityof Illinois, Urbana. ASHRAETransactions: Symposia 505 5000 4000¯ ~ 21"C 3000" 2000 ¯ 1000’ East Wall Ceiling 0 10 30 50 7O 9O 10 30 ~ 70 90 II0 ACH Figure 2a Ventilative cooling rate for sidewall inlet, Figure 1 Ventilative cooling rate for sidewall and three inlet temperatures ceiling inlets--21 °C 8000- flow rate and inlet temperature were varied for each inlet location; 7 tests with reduced inlet area to examine the ----’O---, 16"12 / effects of jet momentum;3 tests with the sidewall inlet diverted awayfrom the north wall; and 4 tests with furni- 6000 ture in the room. The variables and levels for the paramet- ric tests are shownin Table 1. The interior surfaces of the roomwere maintainted at 86°F (30°C) for all tests. Due to the voluminous amount of data obtained, the 4000’ scope of results that cart be presented here is limited. This paper will focus on the subset of results directly related to choosing the best reference temperature and the develop- ment of con’elations for convective heat transfer. Other 2000’ experimental results that cannot be adequately covered here include airflow contour plots, temperature profiles, flow visualization, distribution of convective heat flux aroundthe room, and correlations for individual panels and walls. 0 Thesetopics are included in the final project report (Peder- 10 30 50 70 90 110 sen et al. 1990) and in individual theses (Fisher 1989; ACll Menne1989; Spitler 1990). Figure 2b Ventilative cooling rate for ceiling inlet, three Overviewof Results inlet temperatures Figures 1 through 5 represent the total heat transfer area; second, the ceiling inlet jet impinges on a larger from the roomfor various conditions and flow rates. Figure surface area than the sidewall inlet jet. 1 shows the ventilative cooling rate for each of the two Figures 2a and 2b show the variation in ventilative inlets with varying flow rates but a constant inlet tempera- cooling rate with inlet temperature for each inlet configura- ture of 70°F (21°C). tion. As expected, lower inlet temperatures result in higher For both constant area inlet configurations, with a ventilative cooling rates. Again,all ventilative cooling rates constant inlet temperature, the ventilative cooling rate is are approximately linear with respect to volumetric flow approximately linear with respect to volumetric flow rate. rate. The ventilative cooling rate curve for the ceiling inlet Figure 3 shows the effect of changing the jet momen- configuration is both greater than and has a greater slope tum on the ventilative cooling rates. The jet momentumwas than the cooling rate curve for the sidewall inlet configura- changed by reducing the inlet area. The percentages given tion. This is due to two factors: first, the ceiling inlet has in the legend refer to the percentage of area open. For a higher htlet flow velocities due to its smaller effective inlet constant volumetric flow rate, the 33 %opening has inlet velocities three times as high as the 100 % opening. Like- wise, the 67 %opening has inlet velocities 1,5 times as high TABLE 1 as the 100%opening. Variables and Levels for Parametric Tests As expected, for constant volumetric flow rate, the ventilative cooling rate increases as the inlet area decreases. Variables Levels (For constant volumetric flow rate, inlet jet velocity, 0VolttmcwicFlow Rate (ACI , 15.30.50,70.100 momentum,and energy are all inversely proportional to inlet area.) Also note the "convergence" of the three Inlet Loc~on Ceiling, Sidewall curves at 15 aeh. This implies that at low flow rates, the 506 ASHRAETransactions: Symposia 400O 30OO 10 30 50 70 90 110 10 30 50 70 90 110 ACH ACH Figure Ventilative cooling rate for different size Figure 4 Ventilative cooling rate for diverted and undi- sidewall inlets--21 °C verted sidewall inlets--21 °C effects of jet velocity, momentum,and energy become small comparedwith the effects of buoyancy. Visualization 40o0 of the flow showedthe inlet jet "pouring" into the room and dissipating along the floor at low flow rates (Menne 1989). 30o0 Figure 4 shows the effects of diverting the sidewall inlet jet awayfrom the north wall and toward the center of the room. As shown,diversion of the jet has little effect on the total ventilative cooling rate.
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