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Isobutane flows through electronic expansion valves with hydraulic diameters smaller than 0.19 mm are analyzed in this paper. First, three similar valves but with different orifice sizes were tested with nitrogen to establish the operational envelope in terms of volumetric flow rate and opening ratio. Two valves suitable for household refrigeration were then selected and tested with isobutane under conditions typically observed in practice. To this end, a purpose-built set-up capable of operating with condensation and evaporation temperatures between 25 and 45°C and −28 and −10°C, respectively, was constructed. In order to check for choked flow, performance curves for the mass flow rate were obtained for several opening ratios by varying the backpressure up to values closer to the condensation pressure. The nitrogen results revealed that the largest orifice valve can behave similarly as 3 m long capillary tubes with inner diameters between 0.40 and 0.85 mm. Concerning the isobutane tests, it was found that the valves are nearly insensible to the inlet sub-cooling and that the flow is likely to be choked for household refrigeration conditions. It was observed that the choking pressure ratio is not affected by the inlet pressure, being a function basically of the valve opening. Finally, a π-type correlation was proposed for the flow coefficient, putting forward a model for predicting the valve mass flow rate. An agreement between experiments and model predictions was achieved with 94% of the 138 points within 15% error bounds. [ABSTRACT FROM AUTHOR]

This paper comes out with an experimental investigation of the expansion of refrigerant HFC-134a through adiabatic capillary tubes with internal diameters ranging from 0.23 mm to 0.53 mm aiming at the so-called mesocooling applications. To this end, an experimental apparatus was designed and constructed to emulate the operating conditions of mesoscale refrigerating devices, with condensing pressures ranging from 9 to 19 bar, and inlet refrigerant subcooling ranging from 5 to 10 K. The tube length was varied from 1 to 2 m. The tests were carried out following a factorial design. The experimental data were used to assess the mass flow rate prediction capabilities of an explicit, algebraic model for adiabatic capillary tubes devised in a prior work. The results revealed that the model is not suitable for small-bore capillary tubes. Correction factors were proposed, so that the model predictions for more than 90% of the experimental dataset fell within ±10% error bounds. [ABSTRACT FROM AUTHOR]

Abstract: A first-principles model for simulating the transient behavior of household refrigerators is presented in this study. The model was employed to simulate a typical frost-free 440-l top-mount refrigerator, in which the compressor is on–off controlled by the freezer temperature, while a thermo-mechanical damper is used to set the fresh-food compartment temperature. Innovative modeling approaches were introduced for each of the refrigerator components: heat exchangers (condenser and evaporator), non-adiabatic capillary tube, reciprocating compressor, and refrigerated compartments. Numerical predictions were compared to experimental data showing a reasonable level of agreement for the whole range of operating conditions, including the start-up and cycling regimes. The system energy consumption was found to be within ±10% agreement with the experimental data, while the air temperatures of the compartments were predicted with a maximum deviation of ±1°C. [Copyright &y& Elsevier]

Abstract: This study presents a first-principles mathematical model developed to investigate the thermal behavior of a plate-type, roll-bond evaporator. The refrigerated cabinet was also taken into account in order to supply the proper boundary conditions to the evaporator model. The mathematical model was based on the mass, momentum and energy conservation principles applied to each of the following domains: (i) refrigerant flow through the evaporator channels; (ii) heat diffusion in the evaporator plate; and (iii) heat transmission to the refrigerated cabinet. Empirical correlations were also required to estimate the shear stresses, and the internal and external heat transfer rates. The governing partial differential equations were discretized through the finite-volume approach and the resulting set of algebraic equations was solved by successive iterations. Validation of the model against experimental steady-state data showed a reasonable level of agreement: the cabinet air temperature and the evaporator cooling capacity were predicted within error bands of ±1.5°C and ±6%, respectively. [Copyright &y& Elsevier]

• Novel mathematical model to predict the heat transfer rate of skin condensers. • Experimental validation with eigth hot-wall condenser designs. • Adhesive tape has the strongest impact on the heat exchanger performance. This paper comes out with an investigation of the thermal performance of skin-type hot-wall condensers for household refrigeration applications. Eight different hot-wall condensers were manufactured and installed in identical refrigerators. The condenser samples were constructed by varying the following design parameters: (i) adhesive tape (aluminum or polyethylene), (ii) tube O.D. (4.00 or 4.76 mm), (iii) total length (10 or 11.5 m) and (iv) condenser positioning. The refrigerators were placed inside a climate-controlled chamber and tested in steady-state conditions. A mathematical model that takes into account the heat transfer not only to the surroundings but also to the refrigerated compartments was also put forward. The model predictions were compared to the experimental data when deviations within the ±10% bounds were achieved. It was found that the adhesive tape thermal conductivity and the tube-outer sheet contact area have the strongest impact on the condenser performance. It was also found that the polyethylene tape increases the condensing pressure, due to its low thermal conductivity and thus to the smaller useful heat transfer area. It was thus concluded that the refrigerator foaming process must be carried out with an extreme care to avoid any negative impact on the tube-outer sheet contact resistance. Finally, it was noted that the heat transfer to the refrigerated compartments is directly affected by the condenser positioning and the thermal insulation thickness. [ABSTRACT FROM AUTHOR]

Abstract: This paper outlines an experimental and theoretical study on pulse-width modulated (PWM) flow of HFC-134a through serial expansion valve/capillary tube arrangements. The influence of the operating conditions (inlet pressure, duty cycle, and pulse period) and the geometry (valve diameter) on the refrigerant mass flow rate was experimentally evaluated using a purpose-built testing facility with strict control of all relevant variables. Several experiments were carried out and the results used to calibrate an algebraic first-principles mathematical model. A good agreement between the experimental and calculated data was achieved, with more than 85% of the data points falling within the ±15% error band. It was also observed that the model followed the experimental results closely. [Copyright &y& Elsevier]

Abstract: This paper advances a study of the transcritical expansion of carbon dioxide (R-744, CO2) through adiabatic capillary tubes. The influence of both operating conditions (inlet and exit pressures, inlet temperature) and tube geometry (capillary diameter and tube length) on the CO2 mass flow rate was experimentally evaluated using a purpose-built testing facility with a strict control of the measured variables. A dimensionless correlation to predict the refrigerant mass flow rate as a function of tube geometry and operating conditions was developed. In addition, a theoretical model was put forward based on the mass, energy and momentum conservation principles. The model results were compared with experimental data, when it was found that the model predicts 95% of the measured refrigerant mass flow rate within an error band of ±10%. The model was also employed to advance the knowledge about the transcritical carbon dioxide flow through adiabatic capillary tubes. [Copyright &y& Elsevier]

Abstract: This paper outlines an algebraic model for simulating the transcritical expansion of carbon dioxide through adiabatic capillary tubes. The model was put forward based on the analytical solution of the momentum conservation equation assuming an isenthalpic expansion process. The theoretical model predictions were compared with 66 experimental data points covering different operating conditions and tube geometries. A good agreement between the experimental and calculated mass flow rates was achieved, with more than 94% of the data points lying within an error band of ±10%. [Copyright &y& Elsevier]