Search

Abstract: Epoxy coatings with good adhesion characteristics have been developed that are suitable for large-scale manufacturing and application in compact heat exchangers. Two of them, with a static contact angle for water of 105° and 79° have been tested in a finned-tube condenser comparing flat plate minichannels on the gas-side. Contrary to the expectation, the thermal efficiency of the 105° condenser is slightly less than that of the 79° one. This is due to a reduction of condensate drop size at detachment, resulting in relatively small dry paths after drainage. In the present study, geometry and coating thickness of the two coated exchangers used have not been optimized, since emphasis has been on the effect of contact angle. The finned-tube geometry is shown to be less favourable, for a condenser, than the compact cross-flow plate geometry with minichannels on both the gas- and coolant side that was tested previously. [Copyright &y& Elsevier]

The paper presents a new design of a falling film absorber that could considerably reduce the absorber size without penalizing the vapor and coolant side pressure drops. The proposed design is based on the fundamental characteristics of an efficient absorber design: large liquid-vapor interfacial area and good wetting characteristics. A finite difference scheme is developed to numerically investigate the performance of the design. The results indicate that the proposed design is more compact and efficient than the horizontal tube-type falling film absorber. [ABSTRACT FROM AUTHOR]

In the paper, the design optimisation of a heat exchanger is discussed, using a thermoeconomic approach. The investigation is referred to the tube-in-tube condenser of a conventional vapour-compression heat pump, with a two-phase refrigerant flowing in the inner tube and a single-phase fluid flowing in the annulus. A cost function to be minimised is introduced, defined as the sum of two contributions: (i) the amortisation cost of the condenser, related to the heat exchange area; (ii) the operating cost of the electric-driven heat pump in which the heat exchanger will work, depending on the overall exergy destruction rate in the system. In the paper, this latter contribution is related to the local irreversibility rate in the condenser, using the so-called structural approach. The optimal trade-off between amortisation and operating cost is therefore investigated, by minimising the above-mentioned cost function. A numerical example is discussed, in which, for a commercial heat exchanger, the design improvements needed to obtain a cost-optimal configuration are investigated. The analysis is carried out for three different refrigerants: R22, R134a and R410A. [Copyright &y& Elsevier]

Experiments were carried out to investigate the pressure drop and heat transfer behaviors of a microchannel condenser. The effects of gravity on the condensation of steam in the microchannels were investigated for both horizontal and vertical cases. For the experimental results, the pressure drop of vertical microchannels in the condenser is lower than for horizontal microchannels. In the case of the horizontal microchannel, as the mass flow rate of steam increases from 0.01 g·s−1 to 0.06 g·s−1, the pressure drop increases from 1.5 kPa to 50 kPa, respectively. While the mass flow rate of steam in the vertical microchannel case increases from 0.01 g·s−1 to 0.06 g·s−1, the pressure drop increases from 2.0 kPa to 44 kPa, respectively. This clearly indicates that the gravitational acceleration affects the pressure drop. The pressure drop of the vertical microchannel is lower than that obtained from the horizontal microchannel. In addition, the capacity of the condenser is the same in both cases. This leads to the performance index obtained from the vertical microchannel condenser being higher than that obtained from the horizontal microchannel condenser. These results are important contributions to the research on the condensation of steam in microchannels. [ABSTRACT FROM AUTHOR]

In a survey of the literature from the last 20 years, 20% of the numerical models used to analyze the performance of adsorption chillers assumed the evaporator and condenser were ideal, with a fixed evaporation temperature and condenser temperature, and ignored interactions between the adsorption bed and evaporator/condenser. Even when the interaction with the evaporator and condenser was included, the other 80% of studies modeled the adsorption bed based on the LPM (lumped parameter method), which ignores the geometry effect and contact resistance of the bed, and thus reduces the accuracy of the analysis. As a consequence, these earlier numerical studies overestimated the system performance of the adsorption chiller. In this study, we conducted a refined numerical approach which avoids these limitations, producing estimates in close agreement with experimental results. Compared with our approach, the models with ideal treatment of evaporator and condenser overestimated COP (coefficient of performance) and SCP (specific cooling power) by as much as 16.12% and 24.64%, respectively. The models based on LPM overestimated COP and SCP by 22.82% and 11.28%, compared to our approach. [ABSTRACT FROM AUTHOR]

• Path number in heat exchanger was conventionally determined at rated load. • Number of refrigerant paths of condenser is determined with part load condition. • Path number optimized with part load leads large increase in heat transfer rate. • Optimal number of path was determined by weighting with load conditions. • The proposed method shows promising energy performance potential. Heat exchangers for heat pump condensers and evaporators are usually designed considering rated load conditions. Residential heat pumps and residential air conditioners operate considerably longer at part load conditions than at rated load conditions. This study investigated the difference between a design considering the part load condition and a design considering the rated load condition in heat exchanger path design. The optimal number of paths for each load condition was selected considering the thermal resistance ratio. Results show that the heat transfer rates of a condenser with the optimal number of paths based on thermal resistance under 84%, 51%, and 33% load conditions were 18.29%, 27.58%, and 42.58% greater than the heat transfer rate of a condenser with the number of paths optimized under rated load conditions, respectively. Considering all load conditions, the optimal number of paths with the maximum average heat transfer rate was derived using the weighting factor of the system performance evaluation criteria. The results demonstrated that the optimal number of paths changed when a part load condition was considered in the heat exchanger design. [ABSTRACT FROM AUTHOR]

• Outlet air temperature and charge time are reduced with rising inlet air velocity. • PCM of high PCT is suitable for long period of work ACU and low PCT is suitable for short rush cooling load. • COP of the ACU in case of PCM 24 E is the greatest and PCM 29 Eu is the smallest. • PCM 26 E has the minimum CST and PCM 29 Eu has the longest CST at 0.96 m/s. • For constant mass, rising PCM plate length reduces charging time and saving power. A study on the thermal energy storage (TES) of phase change materials (PCM) coupled with the condenser of air conditioning unit (ACU) is carried out for PCM 24 E, PCM 26 E, and PCM 29 Eu. This coupling technique is based on using the cold energy storage (CES) of the PCM by the cold ambient air at night to cool the ACU condenser at daytime. The study is performed during charging and discharging of the CES at various inlet ambient air temperatures and velocities to the PCMs plates at day and night times. The mathematical model of the physical model is solved numerically by using ANSYS-Fluent software. The findings reveal that during the discharging process, the exit air temperature from the PCM plates is reduced with increasing the inlet air temperature and velocity. The coefficient of performant (COP) of the ACU coupled with the PCM is decreased with time. It starts by a higher value for low phase change temperature (PCT) than high PCT and after a period this trend is reversed. The time required to charge completely the PCM with CES for PCM 26 E is smaller than other studied PCMs at the same temperature difference between the PCT and the ambient. Decreasing the PCM plate width has the advantage of reducing the solidification time but it reduces the saving power by about 10.5%. The saving power due to using PCM rises with decreasing the ambient temperature and inlet air velocity. It is about 8.25, 8.4, and 8.95% for PCM 24 E, PCM 26 E, and PCM 29 Eu, respectively at ambient temperature 35 °C and air velocity 0.96 m/s. [ABSTRACT FROM AUTHOR]

• A distributed model has been developed for multi-pass parallel flow condensers with condensate-vapor separation. • Mal-distribution of refrigerant flow rates in tubes of a flow pass is determined by the genetic algorithm. • Accurate locations of onset and completion of condensation are determined. • Predictions of heat transfer and pressure drop agree with experimental data within ±20%. Liquid-vapor separation is an advanced technology recently developed enabling significant further heat transfer enhancement for condensers. This paper reports a distributed parameter model, using the ε - NTU method, to numerically simulate heat transfer performance of multi-pass parallel flow condensers with liquid-vapor separation (referred to as MPFCs-LS). For achieving higher accurate results and lower computational time, a segment self-subdivision method is used to locate the positions of onset and completion of condensation. Furthermore genetic algorithm is adopted to determine the refrigerant flow rates through tubes in a flow pass. Relevant empirical correlations are selected for heat transfer and frictional pressure drop in different flow regimes during condensation in microfin tubes. The predictions of the model agree well with the experimental data within ±20%. The model and numerical methods developed in this work for MPFCs-LS are of important value in design and performance optimization of these new advanced condensers. [ABSTRACT FROM AUTHOR]