International Journal of Air-Conditioning and Refrigeration

Enhancement of forced-convection boiling heat transfer by electric field is investigated experimentally. When a high-temperature horizontal filament is immersed in water, a gas sheet is formed around and the above filament due to liquid boiling, in the early immersion process. This gas-sheet markedly decreases the boiling cooling rate of the filament. Here, forced collapse of the gas sheet is attempted by imposing an electric field to enhance the boiling cooling rate, In the experiments, a horizontal platinum wire of 0.5mm in diameter is immersed in pure water under atmospheric pressure, and a DC voltage up to 600V is applied between the wire surface and an electrode made of glass placed 10mm apart. The whole boiling curve is measured under different applied voltages and wire-falling velocities in 0.5 to 2.0m/s range, and at subcooling of 60K. The experimental results show that the electric field is effective in promoting the disintegration of the gas sheet. Under the tested conditions, boiling cooling rate increased two-fold for an applied electric field of 600V/cm. This result shows that the use of an electric field to break up the gas-sheet has resulted in a remarkable increase in the cooling rate at high superheats during initial cooling period, which is even greater than that used in the existing material manufacturing processes by the rapid cooling method, and therefore, this method may contribute to developing new materials.

In this work, the performance of window-type air-conditioning units was improved. The study was achieved by adding a diffuser at the compressor outlet. An experimental investigation was carried out on 2 RT window-type air-conditioning units with controlled environmental zones. According to the compressor-outlet conditions, an ANSYS-fluent program was used to achieve a suitable diffuser geometry for maximum pressure increase. The refrigeration cycle model was programmed in Engineering Equation Solver (EES). The experimental work shows that using a diffuser of (5mm ID $\times$ 9mm OD $\times$ 12mm L) in an air-conditioning system will decrease the required compressor pressure ratio by 6% from the base case with an overall pressure increase of 1.4 bar and a 29% COP increase from the base case$.$

The third-generation refrigerants belonging to hydrofluorocarbons (HFCs) do not contribute to ozone depletion. However, HFCs are listed as greenhouse gases by Kyoto Protocol because of their relatively high global-warming potential (GWP). At present the research is now mainly focused on refrigerants with zero ozone depletion potential (ODP) and less GWP, which are termed as Fourth generation refrigerants. This paper analyzes the advancement in refrigerants, and presented the different options in choosing a refrigerant with respect to international agreements to curb the stratospheric ozone depletion and global warming. The hydrofluoroolefins (HFOs) i.e., fourth generation refrigerants are available in limited quantities and also their performance is not completely tested in different applications. Hence this paper aims at assessing the performance of fourth generation refrigerants in terms of their mass flow rate requirement and COP for a specified cooling load and compared with the existing third generation refrigerants in usage. It is found that fourth generation refrigerants COP is low and mass flow rate and power requirements are high. However, HFO1234ze(E) can replace R134a as its performance is almost similar to R134a with an added advantage of low GWP.

In this study, a two-dimensional numerical model of finned-tube type adsorbers was developed and used to examine heat recovery time to improve the performance of an adsorption cooling system. The optimal heat recovery time, which resulted in the highest COP, was determined for a range of heat source temperatures (60–90${}^{\circ }$C) and cycle times (600–1200s). The introduced heat recovery process enhanced COP, but also reduced SCP. This penalty became more serious when the hot water temperature was low and cycle time was short, which serves as a guideline for when heat recovery should be adopted in a given operating condition.

Fault detection and diagnostics (FDD) can be used to monitor the performance of air conditioners (ACs) and heat pumps (HPs), signal any departure from their optimal performance, and provide diagnostic information indicating a possible fault if degradation of performance occurs. For packaged systems fully assembled in a factory, an FDD module can be fully developed for all units of a given model based on laboratory tests of a single unit. For field-assembled systems, laboratory tests of a representative AC or HP installation can lead to the development of a “back-bone” preliminary FDD algorithm; however, in situ adaptation of these algorithms is required because of installation variations in the field. This paper describes a method for adapting a laboratory-based FDD module to field-assembled systems by automatically customizing the in situ FDD fault-free performance correlations. We validated the developed data-clustering technique with a set of nearly 6000 data points to generate fault-free correlations for an HP operating in the cooling mode in our laboratory. The study evaluated several fault-free feature models and indicated that the use of different order correlations during stages of data collection produced better fits to the data.

Heat recovery ensures optimum usage of the collected energy, and thus, minimizes heat loss for a solar adsorption chiller. Two-bed adsorption chiller with conventional single stage, run by direct solar coupling with heat recovery, has been studied mathematically. In a heat recovery adsorption refrigeration system, to facilitate heat transfer, heat transfer fluid is distributed between two adsorbers maintaining the same mass flow rate. There is no mass transfer between system components during this phase. It is a semi-continuous system performed between two adsorption beds. After completion of desorption/condensation mode, heat transfer fluid is allowed to circulate between the heated desorber and the cooled adsorber. This process distributes some heat of the desorption bed to the adsorber preparing it for the next preheating mode where heat transfer between them is done adiabatically. Consequently, the performance has been checked and a satisfactory increase in the Coefficient of Performance (COP) (approximately 15%) has been detected in the calculated results for the heat recovery operation. It is also observed that the heat recovery process enhances the working hour and overall performances of the solar heat driven adsorption chiller.

This study investigated the evaporation heat transfer and pressure drop characteristics of R32 in a horizontal multiport tube consisting of rectangular minichannels with straight microfins. The heat transfer coefficient and pressure drop were measured for a mass velocity range of 50–400kgm${}^{-2}$s${}^{-1}$ and heat flux range of 5–20kWm${}^{-2}$ at a saturation temperature of 15${}^{\circ }$C. The frictional pressure drop during an adiabatic two-phase flow was also measured for a mass velocity range of 50–400kgm${}^{-2}$s${}^{-1}$ and quality range of 0.1–0.9 at the same saturation temperature. The heat transfer coefficient increased with an increasing quality owing to the increase in forced convection. The dryout inception quality increased with the increase in mass velocity. The effects of heat flux on the heat transfer coefficient were small, except in a high-quality region. The heat transfer coefficient in a multiport tube with microfins was higher than that in a multiport tube without microfins in a high-quality region at a mass velocity of 200kgm${}^{-2}$s${}^{-1}$ and in a low-quality region at a mass velocity of 400kgm${}^{-2}$s${}^{-1}$. The effects of mass velocity and microfins on the frictional pressure drop were clarified. It is suspected that the effects of a microfin on the frictional pressured drop can be considered using the hydraulic diameter. The frictional pressure drop was shown to be in good agreement with previous correlations.

This paper describes a prototype of a linear reciprocating magnetic refrigerator using a gadolinium parallel plate. The new design of the regenerator, installation, and experiment is presented. The regenerator consists of a gadolinium plate and an inlet/outlet section. The thickness and length of the gadolinium plate are 1mm and 80mm, respectively. The gap between parallel plates is 0.1mm. Water is used as the heat transfer fluid. The permanent magnet structure used in the present study can generate a maximum magnetic field intensity of 0.94 T. The effects of surface roughness of the gadolinium parallel plate on the temperature span and cooling capacity are investigated. The results show that there is no significant effect of surface roughness on the magnetic refrigerator performance at a high utilization factor. The results from the present study are important for the design of magnetic refrigerators operating at room temperature.

In this paper, parametric study of wet electrostatic precipitator applied to ducted heat pump system was conducted. For the experiment, a dust collecting module was designed and a prototype was manufactured. The airflow rate, the applied voltages of the charging and collecting part were selected as the main parameters in the limited condition. The dust collection efficiency was measured according to the change of the parameter values and the influence of each parameter was analyzed. As a result, the contribution of each parameter on the dust collection efficiency was analyzed as 22.7% airflow rate, 6.6% applied voltage in charging part and 69.9% applied voltage in collecting part. And it was found that the applied voltage of the dust collecting part had the greatest influence on the dust collection efficiency. The correlation equation was derived using the parameters and the product design was made considering this. This design and analysis process will be helpful in future large-scale design.

In this study, the effect of the structure of an ultrafine cryoprobe on its cooling performance was evaluated experimentally and numerically. To clarify the thermodynamic characteristics of the refrigerant in the ultrafine cryoprobe, three ultrafine cryoprobes with different dimensions were manufactured. Additionally, a phase change flow model was developed to estimate the refrigerant condition in a microchannel and evaluate the cooling characteristics of an ultrafine cryoprobe. For validating the numerical model, the results were compared with experimental data and a suitable empirical correlation for a two-phase pressure drop was determined. By calculating the refrigerant condition in an ultrafine cryoprobe, it is clarified that large pressure drops occur in the inner tubes and the refrigerant becomes subcooled owing to heat exchange between the flows in the inner and outer tubes. The temperature differences for three different cryoprobes are reproduced by the developed model. By changing the dimensions of the tubes comprising the ultrafine cryoprobes in the calculation, the lowest temperature can be determined. Additionally, freezing experiments are conducted, and the importance of temperature and vapor quality in ultrafine cryoprobes is represented in the time variation of the frozen region.