Numerical Investigation of a Forced-Air Cooled Condenser Using 1d-3d-Coupling

To further improve the efficiency of refrigerators and freezers, it is necessary to optimize the whole cooling cycle. With one-dimensional numerical simulation programs, the transient behavior of the refrigeration cycle over more than 24 hours can be calculated. A major challenge here is to find appropriate heat transfer coefficients especially when the heat transfer is mainly influenced by forced-ventilation. In the present paper a refrigerator with a forced-air cooled condenser is investigated. To take the influences of the air flow and the condenser geometry on the simulated heat transfer into consideration, three-dimensional flow simulations are used. Due to superheating, subcooling and the transient behavior, the temperature varies across the condenser. Since the transferred heat strongly depends on the distribution of the temperature across the condenser surface, it would be necessary to run the 3d-CFD-simulation after every time step of the 1-d-simulation. To avoid this tremendous numerical effort, a more efficient method with so-called factors of influence is introduced. These factors of influence are calculated, using the results of around 30 CFD-simulations, which are performed with different temperature distributions. After the determination of these factors, the heat transfer for a given condenser geometry can be calculated for different temperature distributions with a simple algebraic equation. To validate this procedure, CFD-simulations with random temperature distributions are performed and compared with the results of the new method. The developed method has been implemented into the 1d-cycle-simulation. Thus it is possible to consider the geometry of the condenser and the complex flow field in the 1d-simulation without a noticeable increase of the computational effort. Finally the results of the cycle simulation are validated with measurement results

Experimental Investigation of an Optimized Mechanically Assisted Suction Reed Valve of a Hermetic Reciprocating Compressor

Reed valves are widely used in hermetic reciprocating compressors for domestic refrigeration. They are crucial components in terms of efficiency, cooling performance, noise and reliability of the compressor. While reed valves already cause a significant proportion of the thermodynamic losses in fixed speed compressors, they induce even more challenges in variable speed compressors. Especially in variable speed compressors, a further improvement of the reed valve dynamics requires the consideration of a new valve concept. In this work, a new design concept of a mechanically assisted suction valve is experimentally investigated. A mechanically actuated spiral spring generates a variable supporting force on the surface of a conventional reed valve. Experiments are conducted over a wide compressor speed range to determine how the new valve design contributes to the thermodynamic requirements of a variable speed compressor, which are high efficiency from low to medium compressor speed and high cooling capacity at high compressor speed. In addition, acoustic measurements will show the influence of the new design concept on noise, vibration and suction gas pulsation of the compressor

Non-Adiabatic Capillary Tubes In Cycle Simulations

Capillary tubes including a suction line heat exchanger are typical expansion devices used in nowadays domestic refrigeration appliances. To account for their functionality in transient cycle simulations, including the highly transient start-up and shut-down operations, different capillary tube models and their implementation in such a cycle are investigated. These non-adiabatic capillary tube models comprise dimensionless correlations, neural network methods and one-dimensional homogeneous models which stem partly from open literature and from previous work of the authors. The difficulty in application of capillary models during off-design conditions is when two phase flow or even superheated vapor which enters the capillary tube. These conditions are not covered by most of the schemes. In this work a transient cycle simulation including a 1d formulation of the heat exchangers and a semi-empirical compressor model serves as virtual test bench for several capillary tube models. The range of parameters is chosen according to the need of domestic applications using R600a - mass flow rates range between 0 and 5 kg/h at inlet pressures up to 10 bar. The comparison of different implementation strategies is carried out in terms of speed, accuracy and stability in off-design. The predictability of the models is evaluated by steady state experimental data from literature and own experiments as well as an in-house 1d model for the regions where no measurements exist. It is concluded that the direct implementation of the 1d code bears the disadvantage of low speed, whereas common dimensionless correlations lose accuracy off its design point. Neural networks turn out to be a good trade-off between speed, reliability and accuracy

Transient 1D heat exchanger model for the simulation of domestic cooling cycles working with R600a

Generally, domestic refrigerators and freezers are running in non-continuous operation mode most of the time, which is a necessity to match cooling capacity to thermal loads. In currently available domestic appliances this matching is realized either by on/off or variable frequency control of the hermetic compressor, leading to a repetitive and transient change of the system state. On the other hand, if the runtimes of the compressor are longer because cooling capacity demand is high (e.g. pull down cycles, initial operation), steady state operating conditions might be reached. The cycling transients cause losses in system efficiency thus they should be reduced or avoided. To understand the complex transient physical processes and to optimize the cooling system efficiency, it turned out that the use of numerical methods is a promising approach. For this reason, a 1D heat exchanger model, which has been successfully implemented in a domestic cooling cycle simulation tool, is presented in this work. The heat exchanger model is a further development of the model being presented in Berger et al. (2012). The same mathematical framework is used for modelling the evaporator and condenser. In order to compute the void fraction, the pressure drop and the heat transfer special empirical models for evaporation and condensation, which are proposed in literature, have been implemented. Finally, the numerical predictions are compared to experimental data gained from a purpose-built test rig

Numerical Simulation of the 3d Transient Temperature Evolution Inside a Domestic Single Zone Wine Storage Cabinet With Forced Air Circulation

This work is carried out in order to investigate the air flow and temperature stratification in the compartment of a single zone wine storage cabinet with forced ventilation for domestic use. The appliance used in this work has a total gross capacity of approximately 135 liters. A single zone wine cooler should achieve evenly distributed temperature for all bottles inside the appliance. To analyze the temperature distribution, a numerical simulation of the air flow and the temperature field can be very helpful. The numerical simulations are carried out applying commercial CFD (Computational Fluid Dynamic) software using the finite volume method. Therefore, the following assumptions are made: the evaporator is modelled by means of a time-varying but locally constant temperature obtained from measurements, the ambient temperature is constant. Concerning the air flow, turbulent conditions are considered. The energy equation is solved transiently and the flow field is calculated assuming steady-state conditions at specific points in time in order to reduce computing time. Furthermore, the air flow in the air channel behind the rear and the top wall of the compartment, where the fan and the freely suspended evaporator are located, is also simulated. The compartment is investigated for different configurations: firstly, an empty wine cooler only with wooden grid shelves and secondly, an appliance loaded with test packages. Temperature measurements with several thermocouples inside the compartment and the air channel are carried out for each arrangement to verify the results of the numerical simulations

Experimental Study on the Thermal Behavior of a Domestic Refrigeration Compressor during Transient Operation in a Small Capacity Cooling System

Generally, domestic refrigerators and freezers are running in non-continuous operation mode most of the time, which is a necessity to match cooling capacity to thermal loads. In currently available domestic appliances it can be observed, that this matching is mainly realized in two different ways: On the one hand, a simple on/off control of the hermetic compressor is installed in lower priced appliances with limited energy efficiency for the mass market. On the other hand, modern top efficiency class appliances have a variable frequency controlled compressor installed. Both control strategies have a repetitive and transient change of thermodynamic states of the refrigerant in common. For better understanding of these cyclic patterns in terms of internal temperature distribution, a state of the art domestic refrigeration compressor with a displacement of approximately 6 cubic centimeters is integrated in a commercial freezer. The compressor which has an on/off control is equipped with extensive measurement instrumentation. Several temperature probes are inserted and temperatures on surfaces inside and outside the compressor as well as refrigerant temperatures are logged for both cyclic and steady state behavior. Finally, a comparison between transient experimental data and steady-state data from a standardized calorimeter test bench is done

Comparison and Validation of Semi-empirical Compressor Models for Cycle Simulation Application

During development of refrigeration cycle simulations, the modelling of the compressor requires a trade-off between computational speed and level of detail. In that case, the use of semi-empirical models makes sense instead of complex 1-dimensional models which are common in the development phase of a compressor. Also the very low measuring effort for the adaption of these models favours their use. The present work contains a comparison of different semi-empirical models found in the literature which simulate the dynamic performance of reciprocating compressors. These models calculate the refrigerant mass flow rate and the compressor power based on polytropic compression and the formulation of a volumetric efficiency. The model parameters are determined by fitting calorimeter data of several compressors. To validate the transient prediction-capability of compressors by these models, start-up and cycling measurements were carried out and compared with the computed data. Due to the common requirements in cycle simulation tools, a model for the discharge temperature was developed and validated additionally. The goal of the present study is to find a combination of accurate models of mass flow rate, compressor power and discharge temperature which can be applied to cycle simulations of the whole refrigeration system