Heat And Moisture Loading Of A Refrigerator Cabinet During Open Door Conditions

Manufacturers have made substantial gains in energy efficiency over the last two decades. The gains have been based on several small, but cumulative effects including improved compressors, controls, heat exchangers, and cabinets. Continued improvement of refrigerators is dependent on tracking down all manners in which energy is transferred into a cabinet. An experimental investigation of refrigerator cabinets is conducted using model cabinet test sections that cover a Rayleigh number range typical of domestic refrigerators. The “cabinets” are constructed from foamboard insulation with the interior covered with aluminum plates that act as calorimeters. A cavity is heated or cooled, then opened to the surrounding ambient air. Transient temperatures are recorded, which are then related to the heat transfer coefficient over the plate. For cooled cavities, when the cavity surface temperatures are below the ambient dewpoint temperature, the cavity weight is recorded. The addition of water condensation mass to the cavity is used to determine the overall cavity mass transfer coefficient. NOMENCLATURE A: area (m) Greek Symbols C: water concentration (kg/m) α: thermal diffusivity (m/s) Cp: specific heat (J/kg-K) β: thermal coefficient of thermal expansion (1/K) Dab: mass diffusivity from A to B (m/s) β: thermal coefficient of expansion with concentration F: view factor (m/kg) g: acceleration of gravity, 9.81 m/s ν: kinematic viscosity (m/s) h: heat transfer coefficient (W/m-K) or (m/s) hfg: latent heat of vaporization (J/kg) Subscripts H: height of cavity (m) air: air j: diffusive mass flux (kg/s-m) cond: conduction transfer J: radiosity (W/m) conv: convection transfer k: thermal conductivity (W/m) exp: experimental L: width (m) f: film properties m: mass (kg) init: initial m& : mass flux (g/s) mass: mass transfer MW: molecular weight (kg) num: numerical Nu: Nusselt number plate: AL calorimeter plate P: pressure (kPa) wall: wall (or thickness) through which heat is being q: heat transfer rate (W) conducted Ra: Rayleigh number rad: radiation transfer RH: relative humidity surf: surface T: temperature (°C) INTRODUCTION The purpose of this research is to assess heat and moisture transport into a domestic refrigerator cabinet during open door conditions as well as determine sensible and latent refrigerator cabinet loading caused by objects removed and replaced into a refrigerator cabinet. The goal is to know how much water forms on the walls of the refrigerator when one opens the door. The air inside a refrigerator generally has lower water vapor pressure than the outside surroundings. When the door is opened, water vapor enters the cabinet; this water ends up on the evaporator in the form of frost. In addition to the energy load caused by moisture condensation, removal of the frost requires energy. The analytical and experimental study of heat/mass transfer in an open cavity is of interest not only in refrigerator cabinets, but also in other areas such as solar receivers, buildings, and electrical components. CASE STUDY DESCRIPTION Measurement Equipment For this project, test cavities were designed using polished 6061-T6 aluminum plates to act as calorimeters (similar to work of M.R. Laleman (1992) and L.N. Knackstedt (1995)) to measure the heat transfer coefficient on the walls of the cavity. The dimensions of the aluminum plates are: 15.24 cm length, 15.24 cm width, and 0.3175 cm thick. Two holes were drilled in each plate at a 45° angle. Thermocouple wire (25 gauge copper-constantan) was inserted in each hole. Thermal epoxy was placed over the lead wires to hold the thermocouple bead in place and ensure good thermal contact between the wire and the plate. After the thermal epoxy cured, quick setting epoxy was then placed over the thermal epoxy for additional strength. Three cavities were designed with various sizes (one plate per side, four plates per side, and nine plates per side). This allows a wider range of Rayleigh numbers to be investigated during testing of the cavities. The larger cavities also allow for the addition of “shelves” to simulate a refrigerator cabinet. The calorimeter plates were sealed to two-inch thick Styrofoam insulation with a thin layer of silicone gel. All plates were separated with 0.635 cm spacing to ensure no physical contact between plates. Silicone gel was also used between the plate spacing to smooth the surface. Heat Transfer Testing on Cavity (no Mass Transfer) A series of tests have been performed on the test cavities for comparison of results to those reported in the literature. Figure 1 shows different cavity orientations examined. The tests first involved placing a cover over the opening of the test cavity to seal the cavity from the ambient environment. Incandescent lights were positioned on the inside cover to warm the inside aluminum plates of the cavity. Fans were also located on the cover to help circulate air throughout the test section to create uniform temperature conditions. Once the desired cavity temperature was achieved, the cover was removed and the cavity was allowed to come in contact with the ambient air.