J. K. Dabas

Bio: J. K. Dabas is an academic researcher from National Dairy Research Institute. The author has contributed to research in topics: Refrigerant & Pressure drop. The author has an hindex of 2, co-authored 4 publications receiving 20 citations.

Performance Characteristics of "Vapour Compression Refrigeration System" Under Real Transient Conditions

Abstract: The behavior of performance parameters of a simple vapour compression refrigeration system were studied while its working under transient conditions occurred during cooling of a fixed mass of brine from initial room temperature to sub-zero refrigeration temperature. The effects of different lengths of capillary tube over these characteristics have also been investigated. It was concluded that with the constantly falling temperature over evaporator, refilling of it with more and more liquid refrigerant causes multifold increase in heat transfer coefficient which helps in maintaining refrigeration rate at falling temperature. Larger capillary tube decreases the tendency of refilling of evaporator but offers less ‘evaporator temperature’ effective in lower range of refrigeration temperature. Shorter capillary tube ensures higher COP initially but which deteriorates at a faster rate in lower temperature range. Capillary tube length must be optimized for maximum overall average COP of the system for the complete specified cooling job.

Experimental investigation of a refrigeration system working under transient conditions

Abstract: The impact of transient conditions along with varied capillary tube length and charge quantity over the performance of a simple refrigeration system under all time transient operations has been investigated in a specially designed experimental setup. A maximum drop of 75% in the coefficient of performance (COP) of the system was recorded by the end of the transient cooling period. The continuous deterioration in performance from start to end of the transient cooling job can be well minimized by the optimum selection of capillary tube length and charge quantity. This paper refers some of the existing methods to determine the appropriate length of the coiled capillary tube and charge quantity for a newly designed refrigeration machine working under steady state conditions and compares the experimental results of transient operation with these. Optimum charge quantity for transient operation in the present study is 3.5% to 5% less than that calculated by the existing analytical and numerical methods. The optimum length of coiled capillary tube for transient operation as found in this experimental study matches approximately with the length predicted by the existing dimensionless correlation on the basis of design parameters as estimated towards the end of the transient cooling period.

Modeling of a Cylindrical Shell and Helical Tube Condenser of HFC-134a

Abstract: A computer program was developed for the performance analysis and design optimization of a cylindrical shell and helical tube type HFC134a condenser and its predicted results were verified against the experimentally determined data. The computer model is based on a numerical method of cell discretization. The local values of variables like heat transfer rate, pressure drop, and the properties of refrigerant are calculated on the basis of appropriate theoretical and empirical correlations available in the literature and the mass, momentum, and energy balance is applied to each cell. The whole sequential and iterative procedure to satisfy the boundary conditions of each cell and of the whole condenser has been transformed into a computer program written in C++. This computer model was used in a parametric study to analyze the effects of varying the input parameters of both fluids on the performance of the condenser. It gives the optimal values of refrigerant mass velocity and of the tube diameter against the available conditions of external cooling fluid, mass flow rate of refrigerant, and degree of subcooling of the refrigerant at the condenser outlet. Its utility was found in the performance optimization of an existing condenser as well as in the design optimization of a new condenser.

MODELING OF A HELICALLY COILED HFC134a EVAPORATOR

Abstract: A computer simulation model has been developed for the performance analysis and design optimization of a helical coil and cylindrical shell type evaporator working with HFC-134a by using the appropriate empirical correlations of heat transfer coefficient and pressure drop in the helical tubes as available in the literature. This model is based on a numerical method of cell discretization of shell side and tube side of the evaporator. The local values of variables are calculated and the mass, momentum and energy balance is applied to each small cell. The whole sequential and iterative procedure to satisfy the boundary conditions has been transformed in the computer codes. The model has been validated by comparing with the actual results of an experimental study which is also a part of this work. A detailed analysis of the effects of varying input parameters of both fluids on the performance of evaporator was carried out with the help of this model. It also gives the optimum values of mass velocity of refrigerant and the helical tube diameter against the available flow conditions of refrigerant and of external fluid and the required degree of vapor superheat at the exit of evaporator. Thus it provides an easy solution in both the cases of either the performance optimization of an existing evaporator or the design optimization of a new evaporator. The inherent errors in the outcome of correlations for heat transfer, pressure drop and refrigerant properties are the limitations of this model.

Refrigeration And Air Conditioning

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A Review of Prediction Methods for Two-Phase Pressure Loss in Mini/Micro-Channels

Abstract: Previous methods and correlations for predicting two-phase frictional pressure loss in mini/micro-channels are reviewed and compared. The empirical correlations are classified into four groups of modeling approaches: Homogeneous equilibrium models (HEMs), separated flow models (SFMs), direct empirical correlations, and flow pattern specific correlations. In order to examine the characteristics of the predictive methods for two-phase pressure loss in mini-channels and to assess the accuracy of the previous models and correlations, extensive experimental data and correlations that are available in the open literature are collected. The 1175 and 1304 experimental data for the two-phase pressure drop for condensing and boiling flows, respectively, are gathered from 15 papers and reports. The results present that the size of the channel significantly influences the pressure drop. The comparison demonstrates that Cicchitti et al.’s two-phase viscosity model is recommended for predicting two-phase pressure loss when the HEM is used. In general, the SFM with the two-phase multipliers of Muller–Steinhagen and Heck and Kim and Mudawar outperforms others for channel diameters of less than 3mm.

Development of high efficiency cycles for domestic refrigerator-freezer application

Abstract: High efficiency 2-stage vapor compression cycles are developed for domestic refrigerator-freezer application. From the parametric analysis of the newly developed cycles, it is recommended that evaporators be connected in series while compressors be connected in parallel to obtain the highest COP (coefficient of performance) in domestic refrigerator-freezer systems. It is found that the most important parameter for performance improvement is the freezing load ratio ( R Q ˙ F = Q ˙ F / Q ˙ t o t a l ). It is also found that the degree of subcooling by the heat exchanger ( Δ T s u b = T 2 − T 3 ) gives the most significant effect on m ˙ t o t a l . However, there are some difficulties in operating the newly developed cycle such as difficult individual operation of the evaporators. Therefore, this study proposes an advanced novel cycle with the evaporators connected in parallel and without the separator, which is named “two-circuit cycle with evaporating subcooler”. It is finally concluded that the refrigerants pair of (R/F) = (R152a/R600a) be the best candidate with the highest COP of 3.758 for the two-circuit cycle with evaporating subcooler.

Non-equilibrium two-phase refrigerant flow at subcooled temperatures in an R600a refrigeration system

Abstract: A refrigeration system is typically designed such that the refrigerant reaches a subcooled state at the condenser outlet. A series of experiments are conducted to investigate the refrigerant state at the condenser outlet and the capillary tube inlet. The refrigeration system used in the present experiment is operated with R600a. The visual observations, as well as temperature and pressure measurements, demonstrate that R600a flows in a non-equilibrium two-phase state at highly subcooled temperatures. Furthermore, a set of equations is proposed for calculating the specific enthalpy of the refrigerant in a non-equilibrium two-phase state, and this calculation method is verified using experimental measurements. It is found that the thermodynamic property table does not provide an appropriate value of specific enthalpy in non-equilibrium conditions, while the enthalpy calculation method proposed in this work is in good agreement with the experimentally measured values.

Dynamic performance analysis of inverter-driven split air conditioner.

Abstract: Variable speed technology has a great potential for improving energy efficiency and performance of vapor compression refrigeration cycle. Therefore, the study of dynamic characteristics of the refrigeration cycle using this new technology becomes essential. This study experimentally investigates the dynamic behavior of a split R-410A air conditioner comprising variable speed compressor, variable speed condenser fan, three speed evaporator fan and electronic expansion valve. The characteristics of the conditioner have been presented during the startup and shutdown periods. Furthermore, the effect of changing evaporator fan speed during the running of the conditioner has been illustrated and discussed. The influence of evaporator and condenser inlet air temperatures on the power consumption and coefficient of performance of the conditioner has also been investigated. The experimental results show that increasing the condenser entering air temperature by 7 °C, the power consumption has been increased by 18% and the coefficient of performance decreased by 15%.