Analysis of a Water-Cooled Unit under Different Loads

In order to ensure the safe operation of the compressors used in water chillers,in the present study some interlock protections have been added to the related design.These include a low pressure protection,a high pressure protection,an exhaust temperature protection and a differential pressure protection.Some tests have been conducted by tuning the saturation suction and exhaust temperatures of the compressor through adjustment of the cold source outlet temperature and the ambient temperature.The results show that the ambient temperature increases with decreasing device load and increasing fan speed under the same saturated suction temperature;the device refrigerating capacity steps up with augmenting load and dropping saturation exhaust temperature,while it is not greatly affected by the fan speed.Moreover,the Energy efficiency ratio(COP)decreases with the rise of the saturation exhaust temperature.This parameter is not affected much by the device load and fan speed at high saturation exhaust temperature,while it improves on increasing the device load and decreasing the fan speed at low saturation exhaust temperature.

Theoretical and Experimental Analysis of Heat Transfer and Condensation in Micro-Ribbed Tubes

The thermal transmission coefficient for a micro-ribbed tube has been determined using theoretical relationships and the outcomes of such calculations have been compared with experiments conducted using a R1234yf refrigerant undergoing condensation.In particular four theoretical single-phase flow and three multi-phase flow models have been used in this regard.The experimental results show that:the Oliver et al.criterion equation overestimates the experimental results as its accuracy is significantly affected by the specific conditions realized inside micro-fin tubes;the Miyara et al.criterion equation prediction error is less than 15%;the Cavallini et al.approach gives the highest prediction accuracy;the Goto et al.model overestimates the test data.Such results are critically discussed and some indications for the improvement of such models are provided.

Experimental Analysis of the Performances of Unit Refrigeration Systems Based on Parallel Compressors with Consideration of the Volumetric and Isentropic Efficiency

The performances of a refrigeration unit relying on compressors working in parallel have been investigated considering the influence of the compressor volumetric efficiency and isentropic efficiency on the compression ratio.Moreover,the following influential factors have been taken into account:evaporation temperature,condensation temperature and compressor suction-exhaust pressure ratio for different opening conditions of the compressor.The following quantities have been selected as the unit performance measurement indicators:refrigeration capacity,energy efficiency ratio(COP),compressor power consumption,and refrigerant flow rate.The experimental results indicate that the system refrigeration capacity and COP decrease with a decrease in evaporation temperature,increase of condensation temperature,and increase in pressure ratio.The refrigerant flow rate increases with the increase in evaporation temperature,decrease in condensing temperature and increase in pressure ratio.The compressor power consumption increases with the increase in condensing temperature and increase in pressure ratio,but is not significantly affected by the evaporation temperature.

A New Model for the Characterization of Frozen Soil and Related Latent Heat Effects for the Improvement of Ground Freezing Techniques and Its Experimental Verification

The correct determination of thermal parameters,such as thermal conductivity and specific heat of soil during freezing,is the most important and basic problem for the construction of an appropriate freezing method.In this study,a calculation model of three stages of soil temperature was established.At the unfrozen and frozen stages,the specific temperatures of dry soil,water,and ice are known.According to the principle of superposition,a calculation model of unfrozen and frozen soils can be established.Informed by a laboratory experiment,the latent heat of the adjacent zone was calculated for the freezing stage based on different water contents in the temperature section.Both the latent and specific heat of water,ice,and particles were calculated via superposition of the weight percentage content.A calculation model of the specific heat of the freezing stage was built,which provides both guidance and theoretical basis for the calculation of the specific heat of frozen soil.

Microscopic Simulation of Expansive Soils and Evolution Laws

In this paper,the discrete element method(DEM)is used to study the microstructure of expansive soils.The results of the numerical calculations are in agreement with the stress-strain triaxial test curve that is obtained for a representative expansive soil.Biaxial compression tests are conducted for different confining pressures(50 kpa,100 kpa,and 150 kpa).Attention is paid to the following aspects:deviatoric stress,boundary energy,friction energy,bond energy,strain energy,kinetic energy,and the contact force between grains when the test specimen is strained and to the effect of the different confining pressures on the internal crack expansion.The results of this research show that the cross-section of the specimen is destroyed along the middle part of the specimen itself.When the confining pressure is higher,the impulse is stronger,and this leads to more effective destruction.

A Water-Heat-Force Coupled Framework for the Preparation of Soils for Application in Frozen Soil Model Test

The freezing of soil containing a liquid is a complex transient heat conduction problem involving phase change and release or absorption of latent heat.Existing efforts have essentially focused on theoretical research and numerical simulations.In the present study,the problem is approached from an experimental point of view using the so-called“freezing model test”method.In particular,in order to establish a precise relationship between the model and the prototype,a temperature similarity criterion is derived using the condition of an equal number of Kosovitch.Similarity is also established with respect to other aspects.A similarity criterion for the water field is determined on the basis of relevant partial differential equations.Analogous criteria for the stress field and load are derived using an elastic model.The validity of this approach is experimentally verified.The research results provide a practical and reasonable method for calculating the parameters for preparing model soils.They also constitute a theoretical basis and a technical support for the design and implementation of a water-heat-force similarity coupled framework.

Heat Transfer Enhancement Using R1234yf Refrigerants in Micro-Ribbed Tubes in a Two-Phase Flow Regime

Experiments about heat transfer in the presence of a two-phase flow due to the condensation of a R1234yf refrigerant have been performed considering a smooth tube and two micro-fin tubes.The following experimental conditions have been considered:Condensation temperatures of 40℃,43℃ and 45℃,mass fluxes of 500–900 kg/(m^(2)·s),vapor qualities at the inlet and outlet of the heat transfer tube in the ranges 0.8–0.9 and 0.2–0.3,respectively.These tests have shown that:(1)The heat transfer coefficient increases with decreasing the condensation temperature and on increasing the mass flux;(2)The heat transfer coefficient inside the micro-fin tube is larger than that for the smooth tube;(3)The heat transfer enhancement factors for the micro-fin tube with a fin helical angle of 8°and 15°are 2.51–2.89 and 3.11–3.57,respectively;both are higher than the area increase ratio.These experimental results have been compared with correlations available in the literature:the Cavallini et al.correlation has the highest accuracy in predicting the heat transfer coefficient inside the smooth tube,the related percentage error and the average prediction error are±8%and 0.56%,respectively;for the micro-fin tube these become±25%and 6%,respectively.