1、NASA w N m M I n TECHNICAL NOTE 4 c/I 4 z NASA TN D-3326 - - - - LOCAL HEAT-TRANSFER COEFFICIENTS FOR CONDENSATION OF STEAM IN VERTICAL DOWNFLOW WITHIN A S/S-INCH-DIAMETER TUBE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. MARCH 1966 Provided by IHSNot for ResaleNo reproduction or
2、networking permitted without license from IHS-,-,-I TECH LIBRARY KAFB. NM 0079970 NASA 1“ U-JSLO LOCAL HEAT-TRANSFER COEFFICIENTS FOR CONDENSATION OF STEAM IN VERTICAL DOWNFLOW WITHIN A 5/8-INCH-DIAMETER TUBE By Jack H. Goodykoontz and Robert G. Dorsch Lewis Research Center Cleveland, Ohio NATIONAL
3、AERONAUTICS AND SPACE ADMINISTRATION For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - Price $0.65 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-LOCAL HEAT-TRANSFER COEFFICIENTS FOR CONDENS
4、ATION OF STEAM IN VERTICAL DOWNFLOW WITHIN A 5/8-INCH-DIAMETER TUBE by Jack H. Goodykoontz and Robert G. Dorsch Lewis Research Center SUMMARY Local heat-transfer data were obtained for steam condensing in vertical downflow in- side a tube. A 5/8-inch-inside-diameter by 8-foot-long stainless-steel wa
5、ter-cooled tube was used as the test condenser. The coolant flowed counter currently in the sur- rounding annulus. Complete condensing occurred in the test section. The downstream vapor-liquid interface was maintained inside the tube in all runs by throttling the conden- sate flow at the exit. Axial
6、 variations of the local condensing heat-transfer coefficient are presented. High values (some in excess of 6000 Btu/(hr)(sq ft)(OF) occurred at the vapor inlet decreasing with distance down the tube to values below 300 Btu per hour per square foot per OF at the downstream end of the condensing sect
7、ion. The local condensing heat-transfer coefficients were strongly dependent on the local vapor flow rate. The mean condensing heat-transfer coefficients for the entire condensing region varied from 2090 to 790 Btu per hour per square foot per OF and showed an approximate linear relation with the to
8、tal mass velocity of the test fluid. Axial temperature distributions for the test fluid, condenser tube wall, and coolant are also presented. The measured axial tempera- ture profiles of the vapor agreed closely with the local saturation-temperature profiles obtained from measured static pressures w
9、hen the inlet vapor was near saturation condi- tions. However, temperature measurements made with the inlet vapor in a superheated state showed that the core of the vapor could remain superheated the entire length of the two-phase region, although condensation was occurring at the wall. INTRODUCTION
10、 Inside-tube-type vapor condensers are being considered for use in Rankine cycle spacecraft power-generation systems. The alkali-metal working-fluid vapors would be condensed within small diameter tubes, and the condensate would flow toward the dis- Provided by IHSNot for ResaleNo reproduction or ne
11、tworking permitted without license from IHS-,-,-charge end of the condenser under the influence of shear and inertia forces. varies with distance down the tube. In cases where the coolant and effective tube-wall coefficients are of the same order of magnitude as the condensing coefficient, the local
12、 heat flux will also vary with length down the tube. Since the rate of heat transfer is closely coupled to the hydrodynamic characteristics of the unit, an improved knowledge of the condensing heat-transfer coefficient is needed to predict the performance of an inside-tube condenser and to optimize
13、the design. Studies of inside-tube condensing have been conducted by a number of investigators (e. g., refs. 1 to 6). However, much of the data reported in these studies is for overall or mean values of the heat-transfer coefficient. Furthermore, where local values are reported, generally only overa
14、ll test-section pressure drops are given so that it is diffi- cult to determine the local vapor pressures. Because of the close interdependence be- tween local fluid-flow conditions and local heat-transfer coefficients, it is important to make both local temperature and local pressure measurements.
15、This becomes partic- ularly important as the mass flow rate and, therefore, the local vapor velocities are in- creased. Hilding and Coogan were among the first to recognize this and a very limited amount of such data is presented in reference 1. data on inside-tube condensing. (near 1 atm) steam, in
16、 addition to being a wetting fluid like the alkali metals, has a liquid to vapor density ratio very similar to the alkali metals at the temperatures contemplated for space powerplant operation. It is therefore likely to have similar hydrodynamic two- phase flow patterns. The insight obtained and the
17、 analytical models formulated from the steam data should be useful as a guide in analyzing the relatively small amount of high- cost alkali-metal- condensing data as it becomes available. local heat-transfer data for steam condensing inside a tube. of condensing heat-transfer coefficients and their
18、variation with distance down the tube were desired. vapor interface was within the tube since this mode of operation would more nearly simu- late an actual space-flight system. The test condenser was a 5/8-inch-inside-diameter by 8-foot-long stainless-steel tube mounted vertically with vapor enterin
19、g at the top and flowing downward. The con- denser was cooled by water flowing upward (counter currently) in an annulus around the tube. The range of variables employed was as follows: In this type of condenser, the local value of the condensing heat-transfer coefficient A program was therefore init
20、iated at the Lewis Research Center to obtain additional Steam was selected as the test fluid. Low-pressure The principal objective of the investigation described in this report was to obtain In particular, local values Special emphasis was placed on obtaining data while the downstream liquid- 2 Prov
21、ided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Test fluid flow rate, wtt, lb/hr . 34 to 117 Inlet vapor pressure, psia 16.2 to 55.8 Inlet vapor velocity, vVi, ft/sec . 65 to 232 Total condensing length, Lc, ft . 5. 9 to 7.7 Coolant flow rate, wk, lb/hr
22、. 1085 to 7680 Coolant temperature, tk, OF: Inlet . 87 to 104 Exit . 96 to 140.5 (All symbols are defined in appendix A. ) Vapor inlet qualities were nominally 100 percent. A few special runs were made with the inlet vapor having superheat up to 17O F. APPARATUS AND PROCEDURE Description of Rig The
23、test facility (fig. 1) consisted of two closed loops that use demineralized water as the working fluid in both loops. The equipment in the vapor loop consisted of a pot- type boiler, superheater, test section, flow-rate-measuring station, receiver, and boiler feed pump. The boiler was equipped with
24、a wire mesh screen and baffle-type separator to remove liquid droplets. The coolant-loop components included a variable-speed pump, flowmeter, heat exchanger, and expansion tank. Building supply steam at 100 pounds per square inch gage was used as the heat source, and cooling-tower water was used as
25、 the final heat sink. The single-tube test condenser was a coaxial-type shell and tube heat exchanger with vapor condensing inside the inner tube and cooled by water flowing in the annulus between the inner and outer tubes (fig. 2). with vapor entering at the top and coolant flowing counter currentl
26、y in the annulus. inner tube was stainless steel with a 3/4-inch outside diameter, a 5/8-inch inside diame- ter, and a total length of 8 feet. The outer jacket was a 2-inch schedule 40 pipe that gave an annular gap width of 0. 66 inch. A low-velocity plenum chamber and an adiabatic re- gion were loc
27、ated upstream of the condensing section. The plenum chamber consisted of a cylinder y4 inches (i. d. ) by 3 inches long with the entrance to the condenser tube being essentially sharp edge. A distance of “16 inches between the plenum chamber and con- densing section comprised the adiabatic region an
28、d consisted of a dead-air space between the condenser tube and coolant exit chamber. The dead-air space was constructed by placing a 13 -(o. d. ) by 0.065-inch-wall tube around the main condenser tube and sealing both ends. additional dead-air space (fig. 2). insulating air space between the condens
29、er tube and the coolant inlet chamber. The test section was mounted in the vertical position The 3 11 1 The vapor plenum chamber was insulated from the coolant exit chamber by an The downstream end of the test section also had an 3 Provided by IHSNot for ResaleNo reproduction or networking permitted
30、 without license from IHS-,-,-Mixing rings were located in the annulus between the condenser tube and outer jacket so that the measured temperature of the coolant would approximate the bulk temperature. The first ring was positioned 8 inches from the start of the condensing section and there- after
31、at 1-foot intervals and mounted to four 1/4-inch rods that ran the entire length of the test section. The test section (shown disassembled in fig. 3) and all vapor lines were lagged with blanket-type insulation to minimize heat losses. A throttle valve was installed downstream of the test section to
32、 assist in controlling the vapor condensing length by throttling the outflow of the condensate. voir for the boiler feed pump. The boiler feed pump was automatically controlled to maintain a constant liquid level height in the evaporator for assurance of steady-state conditions. The receiver was a 4
33、0-gallon tank, partly full during a run, to act as a supply reser- In st ru mentation The test section was provided with instrumentation to measure vapor inlet tempera- ture, vapor and condensate temperature inside the test section, condenser tube wall tem- peratures, coolant temperatures, vapor inl
34、et pressure, static pressure inside the test section at different axial positions, and condensate and coolant flow rates. Figure 4 and tables I and I1 give the axial locations of the test-section thermocouples and test-fluid static-pressure taps. Temperatures were measured with sheathed iron- consta
35、ntan thermocouples. The vapor inlet temperature was measured in the low-velocity plenum chamber upstream of the condensing section. Vapor and condensate temperatures inside the test section were measured by inserting a temperature probe from the exit end of the condenser. The probe consisted of a 0.
36、075-inch (0. d. ) tube with the thermocouple wires inside the tube. The thermocouple junction was held in place at the tip of the probe by a bead of soft solder that also closed off the end of the tube. The probe could be moved to any desired axial position and was kept centered inside the condenser
37、 tube by three equally spaced vanes mounted 1 inch from the end. Condenser-tube-wall temperatures were measured at 4-inch intervals in one longi- tudinal plane. The first thermocouple was positioned 2 inches from the start of the con- densing section. The thermocouple wires were swaged inside a 0.04
38、0-inch (0. d. ) stainless-steel tube insulated with magnesium oxide. The sheath material of the wall thermocouple was stripped back 1/8 inch from the measuring end, thus exposing the thermocouple junction. The bare junction and approximately 1 inch of the sheathed leads were placed in a 0.040-inch-d
39、eep longitudinal groove in the outer wall of the condenser tube. The assembly was then soldered into place, and the surface was ground smooth. 4 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-All wall thermocouple wires left the annular passage at t
40、he coolant exit plenum chamber and were tied to the 1/4-inch rods in the annular passage for support. trates the wall thermocouple installation. the annulus of the test section. The axial locations of the thermocouples are listed in table I. The coolant thermocouples, starting at 0. 51 foot, were po
41、sitioned approximately 2 inches downstream of the mixing rings so that the temperature of the fluid was measured in a well-mixed region. The radial location of each coolant thermocouple was at the mid- point of the annular gap between the condenser tube and outer jacket. In addition, two temperature
42、 measurements were made at the coolant inlet plenum chamber and two were made at the coolant exit plenum chamber. All pressures inside the test section were measured with mercury filled U-tube mano- meters with a hydrostatic water leg on the test-section side and atmospheric pressure on the referenc
43、e side. A horizontal run of bare metal tubing of at least 6 inches was installed between the pressure tap in the test section and the vertical line to the manometer for assurance of an all-liquid hydrostatic head. Vapor inlet pressure was measured in the low-velocity plenum chamber upstream of the c
44、ondensing section with a mercury filled U-tube manometer. Static-pressure taps were placed at the beginning, at the midpoint, and at the end of the condensing section. Condensate flow rate was measured by using a modified weigh-tank technique that consisted of measuring the time required to fill a k
45、nown volume. The temperature of the condensate was monitored so that the flow rate in mass units could be evaluated. The flow-rate-measuring station was located between the condensate throttle valve and main receiver tank (fig. 1) where the line sizes were large enough and the flow rate small enough
46、 that the liquid did not completely fill the cross section of the lines while flowing. The measuring station consisted of a quick shut-off valve positioned downstream of a section of 2-inch (0. d. ) by 3-foot-long glass tube. This arrangement allowed visual ob- servation and timing of the rise of th
47、e liquid interface when the quick shut-off valve was closed. It was assumed that the condensate flow rate did not change when the quick shut- off valve was closed since both the pressure drop across the condensate throttle valve and the pressure in the test section remained constant during the volum
48、etric measure- ment. The coolant flow rate was measured continuously by a commercial turbine-type flowmeter. Figure 5 illus- Coolant temperatures were measured at equal intervals in one longitudinal plane in Test Procedure Before a data run was started, noncondensables were eliminated from the syste
49、m as follows: The evaporator was filled with demineralized water and isolated from the re- 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-maining portion of the system that was evacuated to 28 or 29 inches of mercury through the vacuum line on the receiver. Ten to 15 percent (6 to 9 gal) of the original evaporator inventory was then boil
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