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Hot-spots are a common occurrence in power electronics which become increasingly hotter as chips become denser. Novel cooling technologies are emerging to cope with this increasing heat load, which imbed a condenser to supply cooling drops to the evaporator resting on the hot-spots. Nonetheless, the evaporation process of the drops has been overlooked. Here, we conducted a series of experiments to understand how the evaporation and motion of drops are influenced by the wetting mode of rough hot-spots. We fabricated three different surfaces exhibiting full (Wenzel) or partial (Cassie–Baxter) wetting and the hot-spot is imposed by laser irradiation. We report a direct link between drop motion and wetting mode with the partial wetting drops being highly mobile, attributable to lower pinning energy based on an energy analysis. This study provides a framework for future modifications in hot-spot cooling to account for drop motion which should greatly influence the overall heat removal performance. [ABSTRACT FROM AUTHOR]

HC-290 is a promising alternative for HCFC-22, which is an ozone depleting substance under Montreal Protocol. However, HC-290 charge is to be reduced because of its flammability. The objective of this study was to reduce the charge quantity of HC-290 in a split air conditioner, which was originally designed for HCFC-22. The effect of charge reduction on the performance of the HC-290 split air-conditioning system was studied experimentally by varying condenser tube diameter, length of capillary tube, and condenser air face velocity. Initially, simulation study was conducted using ART simulation software. In accordance with the simulation results, a 5.25 kW split air conditioner was modified and experimentally tested with HC-290 in a psychrometric test facility under standard test conditions as per IS 1391 part 2. The condenser tube diameter was reduced from 12.7 mm to 6.35 mm, while the length of capillary tube was increased from 650 mm to 820 mm with enhanced condenser air face velocities. It was found that with 300 g of HC-290 charge, the performance parameters such as cooling capacity, power consumption, and coefficient of performance were better than the baseline test with HCFC-22 in all conditions tested. [ABSTRACT FROM AUTHOR]

Unexpected fouling in condensers on Central Electricity Generating Board power stations operating on the river Trent in the UK in the 1970s prompted an extensive investigation of the phenomenon. Fouling was caused by deposition of calcium phosphate rich scale on the cooling water side. A fouling test rig was specially constructed to study the performance of different chemical treatments. The rig ran two tubes in parallel, one with undosed water, and the latter's data sets from over 100 "control" tests were analyzed to determine the mechanism and rate of fouling. The trends could not be explained in terms of conventional scaling indices. Insight into the mechanism was provided by separating precipitation (loss from solution) and deposition (adhesion of some of the precipitated mass to the heated surface): scaling was found to be caused by the deposition of particulates in the recirculating cooling water, driven by the change in pH as the water went through the evaporative cooling stage. Quantitative expressions for fouling were generated from tests on 1.83 m long tubes and were found to give a reasonable prediction of the fouling behavior observed on a full sized (18.3 m long) single condenser tube on an operating condenser. The results indicated that recirculating water systems need to be approached very differently to once-through systems, and the use of scaling indices for such systems is not recommended. [ABSTRACT FROM AUTHOR]

The understanding of two-phase flow mechanisms during condensation inside small diameter channels is fundamental to design compact condensers. When dealing with small geometries, experimental investigation can be invasive and the measurement of heat transfer coefficients with low uncertainties becomes difficult. For these reasons, numerical simulations by means of the volume of fluid method are an interesting tool to study two-phase flows and they can also be used in support to experimental investigation. In the present work, first steady-state numerical simulations of R134a condensation inside horizontal channels are presented: the results are used to analyse the effect of the diameter (1 mm and 3.4 mm) on the two-phase flow and heat transfer. Since waves occur at the vapor–liquid interface and they cannot be modelled under the hypothesis of steady-state operating conditions, transient simulations (2-D axisymmetric) have also been performed to investigate the influence of waves on the condensation process. The two-phase flow has also been experimentally investigated and visualizations are compared to the numerical results. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the reduction of energy consumption in a split air-conditioning system employing evaporative cooling of ambient air flowing over the condenser coil. Direct evaporative cooling is employed at the air-cooled condenser of a split air-conditioning system to cool the air flowing over the condenser coils. Different ambient conditions of air were simulated using a heater to mimic typical high temperature environments. The effect of the cooling pad thickness on the performance of the system was investigated by varying the pad thickness from 5 cm to 15 cm in step size of 5 cm. Result shows that the temperature drop experienced by the air is dependent on the thickness of the pad, as well as the condition of the inlet air to the pad. Conditions of the exit air from the pad shows that evaporative cooling can be employed as a stand-alone method for cooling of data centers, with adequate humidity control systems in place, or its output can be used to augment the performance of existing mechanical cooling systems. A decrease in power consumption of the unit is observed, with concomitant increase in coefficient of performance (COP). In addition, results obtained show that up to 44% increase in COP, and a 20% decrease in power consumption can be achieved by employing evaporative cooling. Additionally, the COP was found to increase by about 4% for every 1°C drop in refrigerant condensing temperature. Moreover, a 1°C drop in ambient air temperature causes a drop of 0.6°C in condensing temperature of the refrigerant. [ABSTRACT FROM AUTHOR]

Solar energy can be transformed to storable chemical fuels through high temperature thermochemical processes driven by concentrated solar radiation. Solar thermochemical reactors (STRs) are the key equipment to these processes. An STR integrated with heat pipe technology is proposed. The core part of this reactor is a high temperature special shaped heat pipe (HTSSHP), which is composed of a flat disk-shape evaporator and multiple cylindrical condensers. A second generation HTSSHP with sodium as the working fluid was fabricated and tested in this work. The evaporating behaviors of sodium and isothermal characteristics were investigated at various operation conditions. A temperature pulsation phenomenon on heating surface was observed lasting for a short period during its startup when the heat flux beyond 47.2 kW/m2, implying the generation of film boiling. Further, the operation characteristic of HTSSHP was experimentally proven steady under various heat fluxes and inclination angles, and it showed a good ability to spread temperature at the cooling side where the thermochemical processes take place. This work will facilitate the preliminary understanding of the operation characteristics in HTSSHP. [ABSTRACT FROM AUTHOR]

The aim of this paper is to point out possible solutions to the further development of heat pipes by numerical modelling of phase-change heat transfer. Numerical modelling has now become a standard method that helps to reduce the economic costs of research and development of new and innovative devices. The article describes the procedures for using numerical simulation in phase-change heat transfer that occurs due to evaporation and condensation of the working fluid. The suggested procedure enables modelling of gravitational heat pipe under different changes in its geometry or under changes in the working fluid. From the simulation results we can find out how the heat pipe behaves under different geometric changes and how much heat output it transfers. The paper presents results obtained from modelling of heat pipes, such as classical gravitational heat pipe, heat pipe with capillary geometry, and closed loop pulsating heat pipe. The results gathered from numerical simulation of heat pipes were compared with measurement results. [ABSTRACT FROM AUTHOR]

This paper describes the results of accelerated particulate fouling tests performed on three enhanced tubes and a plain tube. The tests were performed using ferric oxide as the foulant material. Three enhanced tubes included 25 start, 10 start helically ribbed tube and a ripple tube. Effect of the water velocity (1.2–1.7 m/s) on fouling resistance was investigated. The maximum fouling resistance occurred in the 25 start helically ribbed tube (about 8.0 × 10−5m2K/W after 100 hours). For the 10 start helically ribbed tube, the fouling resistance was relatively small (less than 1.8 × 10−5m2K/W). The rippled and plain tubes show almost negligible fouling resistance. High velocity flushing was effective for all the tubes except for the 25 start helically ribbed tube. On-line brush cleaning maintained the fouling resistance below 1.8 × 10−5m2K/W for all tubes. The fouling concentrations used in the tests were significantly higher than would be expected in commercial heat exchangers. Also, the velocity range investigated was lower than would be expected in heat exchanger operation. [ABSTRACT FROM PUBLISHER]

This paper describes the thermal design of a compact vapor compression refrigeration system for high heat flux removal. The system operates with R-134a as the working fluid and consists of a miniature compressor, a compact condenser, and a spray-based heat sink device that combines the expansion device (an array of micro-orifices) and the evaporator into a single unit. The SPHINX (Spray Heat Sink Integrated with the Expansion Device) unit has been designed based on a phenomenological model for the critical two-phase flow in the micro-orifices and phase-change heat transfer resulting from the spray atomization/jet impingement on the heated surface. The SPHINX model was solved together with mathematical models for the other system components (compressor and condenser) in order to obtain the heater surface temperature behavior as a function of the system variables. A parametric study was performed to evaluate the effect of several design variables on the system performance. Correlations available in the literature for spray cooling and two-phase jet impingement heat transfer were analyzed and critically compared. The results indicate that a cooling unit with two compressors in parallel is capable of removing thermal loads of up to 560 W (140 W/cm2) while maintaining the heater surface temperature below 85°C. [ABSTRACT FROM AUTHOR]

A finite temperature difference heat transfer method and irreversibility analysis have been developed for investigating the effects of subcooling on coefficient of performance, cooling water pressure drop of condenser, and heat exchanger area for R1234yf, R1234ze, R22, R134a, and R410A in a single vapor-compression refrigeration system. In order to satisfy the increasing cooling load for subcooling in a condenser, the heat exchanger size or cooling water pumping power that corresponds to initial cost or operating cost, respectively, is increased. The optimum degree of subcooling in a refrigeration system with superior performance and least initial cost or operating cost is obtained numerically. The results show that the maximum coefficient of initial cost saving and coefficient of operating cost saving and their corresponding optimum degree of subcooling increase with condensation temperature. At a higher inlet temperature of cooling water, the optimum degree of subcooling turns out to be smaller for all refrigerants. The results are expected to facilitate the prospective design of a vapor-compression refrigeration system for using alternative refrigerants. [ABSTRACT FROM PUBLISHER]

Conventional modeling of condensers typically assumes three zones: de-superheating, condensation, and subcooling zone, even though it is clear that condensation occurs in the de-superheating zone at some conditions and subcooling occurs during condensation. This article discusses the actual situation and provides experimental validation of the hypothesis. The experimental results show heat transfer coefficients (HTC) of CO2and R410A at mass fluxes from 100 to 240 kg m−2s−1, heat fluxes from 3 to 25 kW m−2, and reduced pressures from 0.68 to 1.00 in a horizontal smooth tube of 6.1 mm inner diameter. Data are compared to correlations proposed for other working fluids or other conditions. Results show much higher values of HTC than the correlation proposed for single-phase turbulent flow in the superheat zone. The occurrence of condensation in the superheat zone is evident when tube wall temperature is below saturation temperature. The results suggest that simplified calculations of heat rejection in a superheated zone could oversize condensers. The semi-empirical correlation, which is here proposed as the combination of existing correlations for single-phase turbulent and saturated condensation, satisfactorily predicts HTC of the superheat zone condensation. [ABSTRACT FROM AUTHOR]

A 0.96 mm circular minichannel is used to measure both heat transfer coefficients during condensation and two-phase pressure losses of the refrigerants R32 and R245fa. Test runs have been performed at around 40°C saturation temperature, corresponding to 24.8 bar saturation pressure for R32 and 2.5 bar saturation pressure for R245fa. The pressure drop tests have been performed in adiabatic flow conditions, to measure only the pressure losses due to friction. The heat transfer experimental data are compared against predicting models to provide a guideline for the design of minichannel condensers. [ABSTRACT FROM AUTHOR]

In this article a numerical investigation is performed on a two-phase closed thermosyphon. Particular attention is given to the condenser section. The working fluid is a binary mixture that consists of two components (R11 + R113) causing a variable conductance behavior in the thermosyphon. The mass, energy, and species conservation equations in conjunction with the overall mass conservation and continuity of momentum at liquid-vapor interface constraints and the thermodynamic equilibrium condition are solved numerically by use of the integral method. The effects of diffusion at the interface of effective and noneffective (shut-off) lengths in condenser are taken into account. The results of the present model are compared with available experimental data and it is found that there is a good agreement with experimental data at low evaporator power levels. [ABSTRACT FROM AUTHOR]

In this investigation, an innovative idea was introduced to design a new kind of high-performance air cooling condensers. This kind of condenser functions to automatically separate liquid from gas and makes condensation always occur in droplet and unsteady thin film condensation mode everywhere in the whole condenser, which results in very high average heat transfer coefficient. An introduction is presented to describe the basic principle and structure of the novel heat exchanger technology, particularly the understanding from the fundamental experimental investigations. Furthermore, a series of experiments was conducted to validate the high performance of an air-conditioning system with an innovative condenser. Though heat transfer area of the condenser was about 37% less, the performance was as good as or even better than that of the original one. [ABSTRACT FROM AUTHOR]

The results of an analytic investigation on the influence of the thermal conductance of a sorption element (adsorber/desorber), evaporator, and condenser on the performance of a three-bed silica-gel-water adsorption chiller are presented with consideration given to the thermal capacitance ratio of the adsorbent and metal of the adsorber/desorber heat exchanger. The analysis was performed by using a cycle-simulation model developed by the authors. The chiller is driven by exploiting waste heat at a temperature 60 and 95°C with a cooling source at 30°C for air conditioning purpose. The results show that the cycle performance is strongly affected by the thermal capacitance ratio and sorption element thermal conductance due to several sensible heating/cooling requirements resulting from batched cycle operation. The model is somewhat sensitive to the thermal conductance of the evaporator, and the thermal conductance of the condenser is the least sensitive parameter. [ABSTRACT FROM AUTHOR]

Results from various enhancement techniques to improve the performance of a recently developed two-phase heat spreader are reported. The spreader has a central evaporator section, with an integrated condenser along the edges. A micro-fabricated three-dimensional enhancement structure is employed to improve the heat transfer performance of the spreader. This study considers the performance of several liquid coolants to be used with the device and evaluates the effect of initial pressures on the thermal performance of the spreader plate. Liquids with lower boiling points were found to result in lower wall temperatures of the spreader plate due to earlier onset of boiling. Studies also showed an improvement in heat transfer performance with increase in stack height of the enhancement structures used in the evaporator section. [ABSTRACT FROM AUTHOR]

In the present work, noncondensable contaminant accumulation in tubes of single-pass, multiple-row, cross-flow condensers is investigated. A mathematical model of condensation in such heat exchangers is proposed, which considers changes along the tubes in the saturation temperature, coolant velocity, and heat transfer effectiveness. The model equations are solved with an original algorithm. The vapor distribution, the global effectiveness of the condenser, and the pressure drop between the inlet and outlet plena are determined under different working conditions. [ABSTRACT FROM AUTHOR]

An experimental investigation has been carried out for the condensation of R-134a vapor on four single horizontal circular integral-fin tubes (CIFTs) made of copper. The fins are of trapezoidal shape and the fin density of these tubes has been varied systematically from 934 fpm to 1,875 fpm. All the experimental data have been acquired at the saturation temperature of 312.4 +/- 0.5 K. The CIFT with 1,560 fpm fin density has been found to be the best-performing tube, as it has provided an enhancement factor (EF) of the order of 5.6. The experimental results are in the best agreement with Honda and Nozu model. This model has predicted 80% of the experimental results in a range of 0-30%. [ABSTRACT FROM AUTHOR]

Proposes modifications to the theoretical solutions for the condensation of multicomponent vapors to obtain a better approximation method to handle more complex mixtures. Different processes for condensing multicomponent vapors; Comparison of the early and presents theoretical solutions for the calculation of the condensation process.