EXPERIMENTAL STUDY AND NUMERICAL MODELING OF NATURAL CONVECTION WITH CONDENSATION Kaushik Das,1 Randall Fedors,2 Chandrika Manepally,1 and Debashis Basu1 1Center for Nuclear Waste Regulatory Analyses Southwest Research Institute® San Antonio, TX 2U.S. Nuclear Regulatory Commission Washington, DC Contact: K. Das, [email protected], Telephone: +1-210-522-4269 Two-phase convective heat and mass transfer I. INTRODUCTION processes in a thermally perturbed, gas-phase dominant environment are studied using an Convective heat and mass transfer in thermally experimental technique and numerical models. The perturbed, dominantly gas-phase environments broad objective of the study is to better understand becomes difficult to assess in geometrically complex the interplay of these processes and to improve the volumes. Use of validated CFD codes for capability to simulate moisture redistribution in geometrically complex problems is generally the thermally perturbed environments utilizing a preferred approach in capturing the flow and thermal computational fluid dynamics (CFD) code. High- field. Radiative, convective, and conductive heat level radioactive waste emplacement in an transfer could all affect temperature distributions. In underground facility is a projected scenario leading addition, the latent heat infusion into the flow field to thermal perturbations. Decay heat load from spent due to the combined evaporation-condensation cycle nuclear fuel and available liquid water at the at the wall boundary and the volumetric flow field repository wall could lead to moisture redistribution, also changes temperature patterns. Essentially, the characterized by repeated condensation-evaporation heat and mass transfer processes affect each other in cycles and driven by natural convection, commonly an interactive manner where the evaporation, referred to as cold trap. In the present study, transport, and condensation depend on the natural laboratory experiments were conducted to convection pattern and overall heat transfer is understand the basic condensation evaporation cycle dependent on the phase change rate. The repeated and provide validation support for numerical condensation-evaporation cycle and natural modeling to simulate heat transfer processes in convection are encountered in a number of concert with moisture redistribution. Within a applications related to high-level waste disposal in rectangular enclosure, evaporation from a heated unsaturated zone, especially in the cold trap process. water source and condensation rate on a cooled wall In this process, seepage water evaporates due to were measured for a range of temperatures. The air radioactive decay heat and condenses on relatively temperature was monitored at specific locations cooler surfaces causing water redistribution in inside the domain. A numerical model was underground openings. Similar physical processes constructed to study the natural convection and also take place in condensers and nuclear power plant phase change using a species transport and components. multiphase mixture model in the CFD package Resolving the conjugate heat and mass transfers ANSYS-FLUENT-12.1. Results of the comparison in a general purpose CFD code requires a modeling between measured and simulated temperature and capability that solves the species transport equations condensation rates are presented; these show in addition to the continuous flow field in a reasonable agreement between the predicted and multiphase environment. The solver also must experimental data. The present study also describes (i) address the issue of phase change at the wall and two different equilibrium models used to calculate the continuous flow field and (ii) track the saturation level in the fluid and their effect on condensate liquid mass that accumulates due to computed results. volumetric phase change. The commercial CFD package ANSYS-FLUENT-12.11 has been adopted to study this coupled multiphase heat and mass transfer The chamber was 23 inches [0.58 m] long, 6 process. The solver has a multiphase flow framework inches [0.15 m] tall, and 12 inches [0.30 m] wide. which was used along with a species transport The tray holding the source water was 3 inches [0.08 module that enabled simulation of evaporation, vapor m] long and ran the entire width of the chamber. The transport, and condensation. User-defined functions top wall of the chamber was made from 1-inch-thick were developed and added to address the issue of acrylic. The left end wall and bottom wall (except for film condensation at the wall and volumetric the water tray, which was made of aluminum) were condensation in the flow field. made of 1-inch-thick [0.03 m] polycarbonate. The Detailed laboratory tests were conducted to front and back walls of the chamber were made of verify the overall modeling capabilities of the general 0.5-inch-thick [0.01 m] polycarbonate. The cold plate purpose solver in predicting the flow and thermal was made of 1-inch-thick [0.03 m] aluminum and field. This well-constrained, small-scale laboratory channels were drilled through it so cooling fluid model was also meant to validate the added could circulate through the plate. Though not shown user-defined modules for calculating the wall and in Fig. 1, the entire chamber was covered with a volumetric evaporation-condensation process before 1-inch-thick [0.03 m], and later a 2-inch-thick [0.06 future modeling of a large-scale experiment with m] shell of polystyrene foam insulation to reduce complex geometry. The experiment was conducted to heat loss. The chamber sat on a table on top of two obtain measurements of water transport rates in a sheets of polystyrene foam insulation. simple two-dimensional natural convection flow. To The water was warmed by an electric heater simplify the validation experiment, low temperatures bonded directly to the bottom of the aluminum water and temperature gradients were used so that radiation tray. The heater pad was the same dimensions as the could be neglected. The present study will first water tray. The heater pad was powered by a variable describe the small-scale experiment and measurement AC supply so that the power level could be varied. results for temperature distribution and condensate Based on the design of heating and cooling rate. Subsequently, the numerical model development components, a two-dimensional convection cell was process will be described and results from this expected to form in the plane following the length analysis will highlight the comparison between the and height of the chamber; temperature measured water transport rates and the rates predicted measurements were made at the centerline of the by the CFD model. width. Thermocouples were installed in the test II. DESCRIPTION OF WATER TRANSPORT chamber to measure the evaporator, condenser, and EXPERIMENTS air temperatures. The locations of the temperature sensors in the test chamber are shown in Fig. 2. The Laboratory tests were conducted to measure the thermocouples were Omega, Type K, model number amount of water transported from a heated water 5TC TT K 30 72 with a bare wire junction and source to a cold (condensation) plate. Multiple tests 0.01-inch-diameter wire. A calibration check showed were conducted by varying the heat rate to the water that all of the temperature sensors were within 0.2 °C source and varying the temperature of the cold plate. of the standard thermometer reading. Once steady-state temperatures were attained for a Water that drained off of the cold plate was test, water transport rates were determined by collected and measured in a graduated cylinder. measuring condensation draining from the cold plate. Depending upon the condensation rate and collection The test chamber is described next. time, different sized graduated cylinders were used to collect the fluid to improve measurement resolution. II.A. Test Chamber II.B. Observations and Test Measurements A schematic diagram and photographs of the test chamber are shown in Fig. 1. The enclosure was Several tests were conducted to measure the made of polycarbonate sheet, acrylic sheet, and moisture transport rate from the water tray at one end aluminum. Water evaporated from the heated tray of the chamber to the cold plate at the other end of shown on the bottom left of the enclosure. Water was the chamber. For each heat rate and cold plate continuously added to the water tray to maintain the temperature, the rate of water condensing at the cold level. Water was condensed on the cooled aluminum plate was recorded after steady-state temperatures plate shown on the right side of the test enclosure. were reached. The test results are shown in Table I. This condensate flow was collected in a graduated The values given in Table I are averages measured cylinder and the rate of collection was recorded. over several hours after steady-state temperatures Cool Plate T Warm Water T Water Water Supply Chiller Electric Heater Condensation Collection Water Source Cold-Wall (a) (b) Fig. 1. (a) Side view schematic of laboratory test chamber and (b) photographs of the chamber; entire chamber and close-up view of source water reservoir 11.5 side walls increased closer to the cold plate. The 2.0 condensation on the sidewalls did not reach all the 4.0 4.0 way to the bottom of the chamber; the bottom couple 6.0 8.0 T1 of inches had no condensation. It was anticipated that T3,T4,T5, T6,T7 T8,T9 the heat loss through sidewalls caused condensation T2 0.1 Top formation on the sidewalls in Tests 1–3. An analysis View of the energy balance was performed considering the Water Tray heater power and the heat loss to the ambient Walls Styrofoam Sheet, 1" Thick environment. The heat loss through the insulation T9 Acrylic Sheet, 1" Thick was estimated using the insulation thickness, End T8 Cold Wall Plate T5 chamber surface area, insulation thermal conductivity 3.0 T7 T4 (0.026 W/m K), and the temperature difference 5.0 1.0 Side 4.5 T1,T2 T3 View T6 measured across the insulation on the top of the chamber.