International Journal of Air-Conditioning and Refrigeration

Borehole heat exchangers (BHEs) are used for transforming a rock mass into an underground heat storage. Usually, their depth does not exceed 200m, but some extend to a depth of almost 3000m. Underground heat storages can operate as part of heating and cooling systems, often economically. In winter they extract heat from the rock mass for space heating, while in summer the cooled rock mass is used for air conditioning. The heat extracted from buildings via air conditioning is transferred into the rock mass, thereby regenerating its condition for winter time. Deep borehole exchangers also may operate only in the heating mode. Then, the rock resource conditions are regenerated via heat transfer through neighboring rocks. If a groundwater flow is present, the heat can also be removed and the source conditions regenerated through convection.

Here, an overview of the use and operation of deep BHEs around the world is provided. Special emphasis is placed on the Carpathians, where numerous analyses of geothermal heat use have been performed since 1999. Examples of calculations for old oil and gas wells as well as negative exploration boreholes are given. Such analyses have been performed for boreholes in Poland and the Ukraine. However, little research has been published on this subject to date, for reasons described herein.

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.

A variable refrigerant flow (VRF) system is widely applied to build air conditioning systems. Literature review of previous VRF modeling studies was conducted, and it was found that the current calculation approach in EnergyPlus can result in more than 20% deviation in both heat transfer capacity and power consumption. Therefore, a new VRF model is proposed in this study, which includes an improved thermal zone distribution concept, new indoor unit types, and a new thermodynamic model. The developed model is validated with experimental data within 10% in heat transfer capacity and 5% in power consumption. Based on the proposed model, the operation control strategy is also discussed. The results show that the proposed model could also simulate the current control strategy in the VRF system.

Cross flow heat exchangers, when applied to cool data center rooms, use external air (process air) to cool the air stream coming from the data center room (primary air). However, an air–air heat exchanger is not enough to cope with extreme high heat loads in critical conditions (high external temperature). Therefore, water can be sprayed in the process air to increase the heat dissipation capability (wet mode). Water evaporates, and the heat flow rate is transferred to the process air as sensible and latent heat. This paper proposes an analytical approach to predict the behavior of a cross flow heat exchanger in wet mode. The theoretical results are then compared to experimental tests carried out on a real machine in wet mode conditions. Comparisons are given in terms of calculated versus experimental heat flow rate and evaporated water mass flow rate, showing a good match between theoretical and experimental values.

The mass transfer enhancement (MTE) by a single emulsion droplet subjected to a Poiseuille flow is studied by using h-adaptive finite element approximation. The Navier–Stokes equations with the surface tension force at a droplet–fluid phase for the fluid flows are considered. The mass transfer is modeled by convection–diffusion equation. The interface capturing method with a conservative level set approach is employed to track the interface of a droplet. This is also incorporated with a mesh adaptivity for increasing the resolution in vicinity of the interface of the droplet. We numerically investigate the effect of hydrodynamic interactions of the droplet with the fluid on the mass transfer in varying physical parameters of the fluid flows and the suspended droplet in order to find the optimum conditions to invoke the enhanced MTE. It is found that Pe is an important factor to enhance the mass transfer transported by the convective current with diffusion. It is also found that Ca is an important factor for MTE considering disturbances and deformability of the droplet during the mass transfer process. It is found that the MTE decreases with increasing Ca in a high concentration region ($C=0.9$) while MTE does not depend much on Ca in a low concentration region ($C=0.1$).

In this paper, the heat pump system has been developed by CFD simulation and experimental investigation. It studies the thermal behavior of a thermobank and COP on heat pump system and cold storage. The thermobank stores the waste heat of during refrigeration cycles and this energy is used for defrosting process and heating room. It also reduces defrosting time and condensation load so that the temperature regulation in the cold storage is constant. The system is investigated experimentally and CFD simulated under thermobank. The dimension $L\times W\times H$ of cold storage is $3\times 1.6\times 1.4$m, thermobank is $600\times 300\times 400$mm. The temperature of ambient on CFD simulation process is 20${}^{\circ }$C. This heat pump system can be used to keep preservation of agricultural products in cold storage warehouse together with floor panel heating for room in winter. The ejector is used in system with the aim of increasing coefficient of performance (COP) and decrease in compressor displacement. As the experimental results, its COP is increased about 38.57% when using thermobank and ejector in heat pump system.

A new refrigerant is needed because the GWP of the R404A and R134a that are used in existing refrigerated trucks and automotive air conditioners is so high. This study predicts cooling performance by using an analytical model, for refrigeration systems using R404A and R134a. Furthermore, the performances of those systems were compared with those of alternative refrigeration systems using R1234yf and R744. The performance data of the analytical model had a trend similar to that of the experimental data: the average error between the analytical and experimental results was within 5.4%. The frost thickness of the R404A system for all operating conditions was about 2.4–3.7% larger than that of other systems because the evaporating temperature of the R404A system was lower than that for other refrigerants. For various operating conditions, the coefficient of performance (COP) of the R134a system was higher than that of other systems, while the R744 system showed the lowest performance. However, the COP reduction of the R744 system with operating time was the smallest because the thermodynamic properties of R744 were very superior. In addition, the frost thickness was seriously affected by the variations of indoor air temperature and compressor rotation speeds, and the system performance decreased significantly for severe operating conditions.

The refrigerant R32 is considered as a potential alternative refrigerant of R410A because of its low global warming potential (GWP) and the basic thermodynamic parameters. It can effectively improve the heating efficiency and reduce the discharge temperature of the compressor to use the economized vapor injection (EVI) refrigeration system. In this paper, the performance of the EVI heat pump system using refrigerant R32 is investigated experimentally under the conditions of the evaporating temperature ranged from $-2^{\circ }$C to $-15^{\circ }$C. The experimental results indicate that the injection ratio, heat capacity and power consumption of the EVI heat pump system increase with the injection pressure. The maximum injection ratio of the EVI heat pump system is about 33% under the evaporating temperature of $-15^{\circ }$C. For the EVI heat pump system, the system COP achieves the maximum value as the injection pressure ranges from 1.48MPa to 1.54MPa when the evaporating temperature is lower than $-6^{\circ }$C. The discharge temperature of the EVI heat pump system drops by 11${}^{\circ }$C under the evaporating temperature of $-15^{\circ }$C. As the evaporating temperature is below $-6^{\circ }$C, the COP of the EVI heat pump system is higher than that of the traditional single stage heat pump.

This research proposed a novel shape design by integrating the geometry that showed the best performance including fin length, pitch and angle on the high temperature heat exchanger of Stirling engine designed for the prime mover of 1kW cogeneration system for home. First, the numerical simulation was conducted on the new design and existing shape, and the performance improvement according to the shape optimization was checked. Next, the validity of its performance was verified by additionally considering the heat loss from the recuperation and low-temperature heat exchanger. As a result, when the high temperature heat exchanger is optimized, a great amount of heat quantity is absorbed from the fuel gas from the upstream part where negative heat flux occurred in the cylinder head part. This was judged to be because of the fixed temperature of the high-temperature part in the thermodynamic cycle. Thus, when researching the shape of the high-temperature heat exchanger, an optimized geometry can be obtained when combining cycle interpretation of system rather than interpreting independently.