NATURAL CONVECTION HEAT EXCHANGERS FOR SOLAR WATER HEATING SYSTEMS. Technical Progress Report November 15,1996 to January 14,1997

Transcription

1 NATURAL CONVECTION HEAT EXCHANGERS FOR SOLAR WATER HEATING SYSTEMS Technical Progress Report November 15,1996 to January 14,1997 Jane H. Davidson Department of Mechanical Engineering University of Minnesota Minneapolis, MN Prepared for the United States Department of Energy Under Contract No. DE-FG36-94GO10030 g':fi*y {' IF' r-7 1,!7i

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4 NATURAL CONVECTION HEAT EXCHANGERS FOR SOLAR WATER HEATING SYSTEMS DE-FG36-94GOlOO30 Jane H. Davidson University of Minnesota Reporting Period: Project Personnel: November 15, January 14,1997 Jane H. Davidson, Faculty; Scott Dahl, Ph.D. candidate Project Objectives: The goals of this project are 1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems and 2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows. 1) Devellop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol., 2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems. Progress;: Task 1 is complete. Task 2 is underway. The primary objective of the experimental work is to determine the type of relationships that are required to fully characterize the pressure drop and heat transfer performance of thermosyphon heat exchangers used in solar water heaters. Preliminary heat transfer and pressure drop measurements for the four tube-in-shell heat exchanger shown in Figure 1 are reported. The thermosyphon water flow is on the shell side of the heat exchanger while the forced fluid from the collector is in the tubes. In this first set of experiments, electric heatiers provide energy to the water. Electric cartridge heaters were inserted inside the heat exchanger tubes. Each electric heater cartridge is cm in length with a cm O.D. and is capable of providing up to 1600 Watts. The water flow was forced and the inlet temperature was controlled. The use of electric heater cartridges and forced water flow allowed for tight control of the important dimensionless parameters on the thermosyphon side of the heat exchanger (i.e. Grashof number, Gr, and Reynolds number, Re). The effect of the Prantltl number, Pr, was investigated by controlling the temperature of the water entering the heat exchanger. The ranges of the dimensionless parameters were, 300<Re<2200, 4.95E+O.kGr<3.30E+O7, and 2.2<Pr<5.9. The values of Re and Gr were limited to the laminar flow regime because this is the expected regime in solar water heating systems. 2

5 Th,l?ID ' 1.59 cm O.D. (0.95 cm 1.D) Brass Tube ' 5.40 an O.D. (5.04 cm I.D.) Copper Tube O.D. Copper Tube y I 0.95 cm Figure 1.. Four tube-in-shell heat exchanger. Tests to determine the thermal and hydrodynamic response times of the heat exchanger to step increases in the heating rate and/or water mass flow rate show that the temperature, flow rate and pressure reach steady state after ten minutes. Reported values are based on measurements obtained after 20 to 25 minutes at each setting. The reported temperature is the average of five consecutive measurements taken over one second while the values of pressure drop and flow rate are averages of twenty consecutive measurements over a five seconds. Figures 2 through 5 present heating rate, flow rate, pressure drop, Grashof number, Reynolds number, and temperatures as a function of time during a test during which the water inlet temperature, Tci, was 25 C. Figure 2(a) and (b) are plots of mass flow rate and pressure drop as functions of time at heating rates of IO00 W and 2000 W, respectively. The water flow rate is controlled using a globe valve and is increased in increments of 0.02 kg/s from 0.02 to 0.08 kg/s. At flow rates below 0.02 kg/s, the shear pressure drop was below the minimum detectable value of our pressure transducer. The pressure transducer (Rosemount Model 3051) has an accuracy of +_0.075%FS and an operating range of 0 to 125 Pa. Fluctuat.ions exist in the pressure drop even though the mass flow rate measurement is stable. The fluctuations increase in magnitude as the flow rate is increased. Heating rate has no influence on the magnitude of the fluctuations. Note that the splkes in pressure drop are due to the sudden step changes in flow rate and are not considered to be fluctuations. 3

6 0.09 -r g 0.02 m g g i i zm 91 r" B E ii Figure 2. Measured water mass flow rate and shear pressure drop as a function of time (a) 0 lo00 Watts (b) Watts. One possible explanation for the fluctuations in pressure is that the flow is in transition from laminar to turbulent flow. Unlike flow in circular tubes, where transition begins at Reynolds numbers around 2100, the flow in tube bundles may enter into the transition regime at Reynolds numbers' as low as 600 to 1000 (Cheng and Todreas, 1986). Suh et al. (1989) also observed fluctuations in pressure drop during experiments with mixed convection flow in a 19 rod hexagon array using electrically heated rods and water-as the heat transfer medium. They used the magnitude of fluctuations in the differential pressure drop measurement as an indicator of transition flow. They assumed fluctuations greater than 1.33 Pa on a time scale of 1 second indicated transition. The Reynolds number at which transition began was found to be between 800 and 1100 for Gr < 3 x lo5 and then decreased to approximately 650 at higher Grashof numbers. Figure 3 presents the Reynolds number and Grashof number as a function of time for our measurements. Comparing Figures 2 and 3, it appears that noticeable fluctuations, 2 2 to 3 Pa, begin at Reynolds numbers of 800 and 1o00. 1ooo:17 b u 1200 ~ 4.OE E w I 3.4E E+06 I 3.OE+06.. f 2.8 E E E+06 -a-re -A Grq 2.4E E+06 2.OE Figure 3. Reynolds number and Grashof number plotted as a function of time. 4

7 8 Another possible explanation for the fluctuations in pressure drop is instability of the instrumentation. Although the magnitude of the fluctuations increases with increasing flow rate, the ratio of the magnitude of the fluctuations to the magnitude of the average pressure drop is 5 to 7% over the entire flow range. Additional experiments are planned to investigate the fluctuations. Spatially averaged temperature measurements at the tube wall, shell wall, and inside the shell are presented in Figure 4. There are ten equally spaced thermocouples on the shell wall and on the exterior tube walls. There are five thermocouple probes with a diameter of 0.51 mm inserted 1 cm into ithe water flow through the shell wall Figure 4.. Heat exchanger temperatures as functions of time. Average water temperature within the shell is within 2 C of the average of the inlet and outlet water temperatures. Thus, it is reasonable to use the average of the inlet and outlet temperatures to evaluate the fluid properties and the hydrostatic pressure correction in the differential pressure drop measurements. The difference between the average shell temperature and the average probe teimperature indicates that a significant error would exist in the differential pressure drop reading without this correction. The measured pressure drop would be less than the actual pressure drop by 1 to 8 Pa, depending on the heating condition. Figure 5 shows a comparison of the electric energy supplied by the heater cartridges to the energy gained by the water. The electric power is measured with a power transducer (Yokagawa Model 2489) along with a 1O:l current transformer. The electric power measurement has an accuracy of % FS and a range of 0 to 4500 Watts. The energy gained by the water is within 6% of the electric power. The difference between the electric energy and the energy gained by the water is attributed to losses to the ambient. Figure 6 shows a comparison of the electric energy and heat transfer to the water at steady state conditions over the full range of heating rates used in the present experiments. 5

8 r' 1 - t Q electric --n-.qwater O J Figure 5. Electric energy supplied and energy gained by the water as functions of time g m OOo U o/ 0 : I OO 3ooo ooo Q eiectric, Watts Figure 6. Comparison of the electric energy supplied to the energy gained by the water at steady state cortditions for the entire range of electric heating rates. Continuing Work: Empirical correlations are being pursued which correctly characterize the pressure drop and heat transfer in the thermosyphon heat exchangers under uniform heat flux conditions. References: Cheng, S-k. and N.E. Todreas, 1986, "Hydrodynamic Models and Correlations for Bare and Wire-Wrapped Hexagonal Rod Bundles - Bundle Friction Factors, Subchannel Friction Factors and Mixing Parameters," Nucl. Engineering and Design, 92: pp Suh, K.Y., N.E. Todreas, and W.M. Rohsenow, 1989, "Mixed Convective Low Flow Pressure Drop in Vertical Rod Assemblies: 11-Experimental Validation," Journal of Heat Transfer, 111: pp It