Vapor Side Research Articles

Experimental investigation of eutectic phase changing materials (KCL + Na2SO4) mixed with graphene (Gr) nano-particles on refrigeration test rig

There has been increasing interest of various researchers in sustainable solutions towards HVAC and R. As we know that Indian refrigeration market has had remarkable growth of around 8.1 % Compound Annual Growth Rate (CAGR) over the period 2021–27. In India, several industries introduced deep freezers, refrigerators, refrigerated trucks for the preservation of different commodities like fruits, veggies, milk, Ice-cream, meat, medicines, etc., at 0 to 15 °C temperature. Thus, an attempt has been given on energy-saving opportunities for power saving by considering the above scope of refrigeration and cold chain applications. In this context, the present research is to prolong the cooling period during a power outage and minimize the losses which are occurred due to temperature fluctuations of the evaporator cabinet of the refrigeration test rig by using Nano-phase change material (KCl + Na2SO4 + Graphene) with water as base fluid. The experimental setup is tested during power ON and OFF conditions and performance is studied on the basis of cooling time, power consumption, and COP. The dispensability of nanoparticles and PCM is observed with the help of Scanning Electron Microscopy (SEM). The experiments were tested for 1 % and 5 % (KCl + Na2SO4 + Graphene) Nano-PCM mixture and it has been found that the addition of graphene nanoparticles in phase change material results in an enhanced heat transfer as well as improvement in 12 % COP. Also, power consumed with the advancement of Nano-PCM in the evaporator side is 10 % lesser than that of the conventional one. It is observed that with the use of 5 % (KCl and Na2SO4) addition of 1 % (Gr) with water as the base fluid it is possible to maintain 7 to 9 degrees for 13 to 15 hrs time span even though a power outage exists. The temperature range 5 to 10 degree is standard temperature for storing the fruits, veggies and medicine/vaccine. It is observed that thermal energy storage by using Nano-phase changing material for lower temperature application is the most appropriate way and method to correct the gap between the demand and supply of energy in cold chain applications.

Low-profile heat pipe consisting of wick-lined and non-adiabatic wickless wettability-patterned surfaces

Heat pipes are metal-wick equipped systems that circulate a phase-changing liquid and transfer heat more effectively than solid-metal compact heat spreaders. Metal wicks are effective fluid transporters, but they can face capillary limits due to significant pressure drops and pore clogging. Wettability-patterned (WP) surfaces, on the other hand, are not limited by these constraints and, as demonstrated in this work, may offer a viable supplement for metal wicks in heat pipes, which come in many shapes and sizes. The use of WP surfaces facilitates the collection and efficient transport of condensate, thus improving heat transfer from the hot (evaporator) to the cold (condenser) side of the device. Most commonly, flat heat pipes consisting of a fully wick-lined plate opposed to a wickless surface, are evaluated considering a uniform adiabatic condition on the wickless plate, which does not promote condensation. However, in a practical scenario, the heat pipe portions in contact with an external heat sink could induce condensation on the wickless side, thus affecting the heat pipe’s operation and thermal resistance. To explore this scenario, a flat heat pipe of 10 cm operating length with axially graded copper-wick evaporator and non-adiabatic wickless WP surface is designed, fabricated and tested. A wettability pattern is imparted on the wickless surface to control the maximum droplet size of the condensate and condensation mode via spatially designed superhydrophilic/hydrophobic juxtaposed areas to regulate both Dropwise Condensation (DwC) and Filmwise Condensation (FwC) towards enhanced heat transfer with reduced overall system thermal resistance. Low capillary pressure domains are allocated in this design to collect condensate on the wickless plate and return it to the evaporator’s wick. On the evaporator side of the system, variable-porosity copper wicks are being used to improve liquid conveyance from the cold to the hot domains. Four heat-transfer fluid charging ratios are examined with the best thermal resistance 0.27 K/W achieved at 105 W. The effect of wettability patterning on the wickless surface and the wick characteristics of the wick-lined evaporator are investigated. • Low-profile hybrid heat pipe comprised of wick-lined and wickless components. • Three distinct wick structures and wettability patterns investigated and compared. • Axially-graded wick structure outperforms uniform wicks. • Wettability-patterned wickless plate lowers thermal resistance. • Highest-performing system’s thermal resistance below 0.3 K/W.

Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump

The hydrogen compression that occurs in a metal hydride compressor is based on the chemical and thermal processes of hydrogen absorption and desorption into an intermetallic structure of metals. The concept presented in this article is centred on the use of an optimal metal alloy that is capable of absorbing hydrogen into its structure at a low temperature and pressure. After such an alloy is heated up, its pressure will significantly increase, and this facilitates the compression of hydrogen without it being in direct contact with the movable parts of the compressor. As a result, the safety of the compression process is increased. The hydrogen compressor contains a pair of tandem metal hydride (MH) tanks with an integrated heat exchanger. The tanks are alternately heated and cooled, so that while hydrogen is absorbed in one of the tanks it is concurrently desorbed in the other tank. The unique nature of the prototype single-stage metal hydride compressor consists in using a heat pump that facilitated a significant reduction of the consumed electric energy in the heat transport between the tandem-arranged tanks. The purpose of this investigation was to examine the possibility of integrating a heat pump as a source of heat and cold for the process of absorption- and desorption-based hydrogen compression. With the prototype MH compressor presented in this article, a compression ratio of ε = 2.0 was achieved while the tanks were operated at temperatures ranging from 12 to 55 °C. In a single cycle, the compressor worked with 166 litres of hydrogen, which was absorbed into 3.125 kg of the La0.85Ce0.15Ni5 alloy. The average heating power that was achieved on the side of the condenser that heated the MH1 tank was 362 W, and the cooling power that was achieved on the evaporator side was 300 W. The achieved value of the heat pump COP was 2.92.

도심 항공 이동수단용 20 kW 급 냉매사이클 설계

This study presents design specifications for a 20 kW refrigerant cycle for the thermal management system of urban air mobility. The secondary-loop whose operating liquid is water is applied to both the evaporator and condenser. The lightweight compressor by the high-speed operation is also applied. The design condition of the refrigerant cycle is based on the refrigerant cycle for vehicle air conditioners using refrigerant R134a. The coefficient of performance (COP) and the specific speed of the compressor are calculated according to various refrigerants, and U·A (total heat transfer coefficient·area) value of each evaporator and condenser side are calculated by the heat equilibrium equation. After reviewing various refrigerants applicable in the future, it was confirmed that R1233zd had an advantageous COP in cooling and heating conditions, and a high-speed centrifugal compressor of 100,000 rpm class can be applied to the R1233zd refrigerant cycle. In addition, the U·A values that determine the size of the heat exchanger are 0.62 kW/K for the evaporator and 1.43 kW/K for the condenser.

Energy and exergy analysis of a novel solar-air composite source multi-functional heat pump

As multi-functional heat pump with higher equipment utilization rate will face more complicated operation condition, the integration with composite heat source will be good solution to improve the operational stability and output performance. In this paper, a novel solar-air composite source multi-functional heat pump (SA-CMHP) has been proposed. Different from conventional single-source heat pump, the proposed system has two evaporators operating in parallel in space heating mode. As the distribution of refrigerant at evaporation side is associated with the output performance of SA-CMHP, the space heating performance under different refrigerant distribution ratio (γ) has been studied based on energy and exergy methods. Besides, economic performance of SA-CMHP for space heating has been analyzed and compared with single-source heat pump. The results indicate that with the increase of γ, the variation of evaporation capacity, power consumption, heating capacity, COP and exergy efficiency tends to increase at first and then decrease. When solar irradiation is 200W/m2, 300W/m2 and 400W/m2, COP and exergy efficiency reach the highest value at γ of 0.6, 0.7 and 0.8. When ambient temperature is 4 °C, 7 °C and 10 °C, COP and exergy efficiency reach the peak values at γ of 0.6.

Investigation on evaporation characteristics of ethanol in annular pool with different liquid layer depths at low pressures

In order to understand the energy transfer process and non-equilibrium effect during ethanol evaporation in its own pure vapor at low pressures, a series of the experimental measurements and numerical simulations are conducted in an annular pool at different liquid layer depths. The results show that the interface temperature of vapor side is higher than that of liquid side when ethanol evaporates in its own pure vapor at low pressures, and the magnitude of interface temperature discontinuity decreases with the increase of liquid layer depth. There are two vortexes below the evaporation interface because of the large radial temperature gradients on the free surface near the cylinders and the buoyancy effect. With the increase of the liquid layer depth, the flow is enhanced and the average evaporation rate increases. In this case, the flow destabilizes from steady state to three-dimensional oscillatory state due to the lag between the variations of velocity and flow resistance, and the petal-like pattern can be clearly observed below the liquid-vapor interface. The evaporation flux near the inner and outer cylinders is much larger than that in the middle region; most of the latent heat required for evaporation is transferred to the interface near the cylinders by thermal convection and the conduction.

Performance optimization of solar-air composite source multi-functional heat pump with different control schemes

Heat pump is a promising approach for low carbon heating/cooling driven by renewable electricity and adapting to multiple heat sources. In this paper, solar-air composite source multi-functional heat pump (SA-CMHP) with the capabilities of space heating, domestic water, space cooling, heat storage, and defrosting has been proposed to provide effective heating/cooling supply solutions for buildings. In SA-CMHP, as the distribution of refrigerant at evaporation side has great impact on its space heating and water heating performance. To optimize its performance under different working conditions, two different control schemes are developed. Mathematic model is established to evaluate and compare the behavior of SA-CMHP under different control schemes. The results reveal that control scheme 1 should be employed when the solar irradiation is insufficient, otherwise, control scheme 2 is more effective for space heating mode. Meanwhile, control scheme 1 should be adopted in water heating mode under different working conditions, as control scheme 1 can effectively reduce the heat loss to environment at evaporation side during the water heating process due to the capability of adaptive regulation of mass flow rate distribution at evaporation side.

Finite size effect on the existence of the liquid–vapour spinodal curve

Fluids interacting with Lennard-Jones and Square Well potentials reach thermodynamic equilibrium forming two coexisting phases when simulated within the liquid–vapour spinodal region. If molecular dynamics simulations in the NVT ensemble are performed on a fluid kept in a cubic cell with periodic boundary conditions, liquid and vapour phases at equilibrium are separated by a process of spinodal decomposition. In the vapour side of the spinodal curve, spherical, cylindrical, and slab type liquid structures are formed, depending on the total density. Under the mentioned conditions, in the liquid side of the spinodal, vapour bubbles with the same shapes, surrounded by liquid, are formed. Results from this work suggest that as the simulation system size increases, the spinodal curve approaches the orthobaric, but it converges to a position closer to the coexistence curve but separated from it. The same thing happens for both potential models simulated. This is in sharp contrast with previous findings by Errington, McDowell, Binder and coworkers, where they conclude that ‘Only for infinitely large systems does the effective spinodal density converge to the macroscopic coexistence vapor density.’

Research on Optimization of collector module of new flat plate heat pipe PV/T heat pump system

The mathematical model of a new flat plate heat pipe PV/T heat pump system is established. The experimental data of the system under various working conditions are obtained through experimental measurement, and the accuracy and reliability of the model are verified. Based on the verified mathematical model, the thermal performance, electrical performance and the performance of the heat pump system are simulated. The results show that under winter conditions, the daily average thermal power, electrical power and COP of the system are 274.5 W, 93.5 W and 2.7 W respectively. Due to the low outdoor ambient temperature in winter, during winter operation, the heat collection system will lose a lot of heat to the surrounding environment through the photovoltaic panel surface, resulting in the heat collection of the system cannot meet the heat demand of the heat pump side, which is intuitively shown as the water temperature of the heat collection tank on the evaporation side shows a downward trend throughout the day. Therefore, the collector module of the system is optimized by adding a collector. After optimization, the daily average thermal power of the system is increased to 654.2 W and the COP is increased to 6.9.

Analysis on frosting of heat exchanger and numerical simulation of heat transfer characteristics using BP neural network learning algorithm.

The study is aimed at the frosting problem of the air source heat pump in the low temperature and high humidity environment, which reduces the service life of the system. First, the frosting characteristics at the evaporator side of the air source heat pump system are analyzed. Then, a new defrost technology is proposed, and dimensional theory and neural network are combined to predict the transfer performance of the new system. Finally, an adaptive network control algorithm is proposed to predict the frosting amount. This algorithm optimizes the traditional neural network algorithm control process, and it is more flexible, objective, and reliable in the selection of the hidden layer, the acquisition of the optimal function, and the selection of the corresponding learning rate. Through model performance, regression analysis, and heat transfer characteristics simulation, the effectiveness of this method is further confirmed. It is found that, the new air source heat pump defrost system can provide auxiliary heat, effectively regulating the temperature and humidity. The mean square error is 0.019827, and the heat pump can operate efficiently under frosting conditions. The defrost system is easy to operate, and facilitates manufactures designing for different regions under different conditions. This research provides reference for energy conservation, emission reduction, and sustainable economic development.

Precooling of fresh air in façade mounted photovoltaic thermal panels by refrigeration

The integration of the photovoltaic thermal system on the building assists in supplying and reducing the building energy demand. The system integrates photovoltaic thermal modules and generates both electrical power and heat for the buildings. The current study focuses on the utilization of waste cooling energy from the photovoltaic thermal evaporator side as novelty in this research area. This will not only add up energy to the building system but also would be thermally efficient, if addressed properly. In this endeavor, we have investigated the performance of fresh air intake precooling in façade mounted photovoltaic thermal panels cooled by a refrigeration system. The structure of the photovoltaic thermal system, its design on the façade and the whole setup is presented for the first time ever, followed by the experimental construction platform. Finally, the ambient fresh air cooling characteristic and electrical power generation is analyzed through performance evaluation indices under natural weather conditions. The experimental results indicated the pre-cooled fresh air temperature difference of 10.27 °C. The maximum electrical efficiency of photovoltaic was 10.16% during testing period. The results can provide a valuable reference for the utilization of photovoltaic thermal panels assisted by heat pump system in supplying fresh air to indoor environment.

Hybrid Cascade Heat Pump and Thermal-Electric Energy Storage System for Residential Buildings: Experimental Testing and Performance Analysis

The need for innovative heating and cooling systems to decarbonize the building sector is widely recognized. It is especially important to increase the share of renewables at building level by maximizing self-consumption and reducing the primary energy demand. Accordingly, in the present paper, the results on a wide experimental campaign on a hybrid system are discussed. The system included a sorption module working as the topping cycle in a cascade configuration with a DC-driven vapor compression heat pump. A three-fluids heat exchanger with a phase change material (PCM), i.e., RT4 with nominal melting temperature of 4 °C, was installed on the evaporator side of the heat pump, for simultaneous operation as thermal storage and heat pumping purposes. The heat pump was connected to a DC-bus that included PV connection and electricity storage (batteries). Results showed that the energy efficiency of the heat pump in cascade operation was double compared to compression-only configuration and that, when simultaneously charging and discharging the latent storage in cascade configuration, no penalization in terms of efficiency compared to the compression-only configuration was measured. The self-sufficiency of the system was evaluated for three reference weeks in summer conditions of Athens climate and it was found that up to 100% of the electricity needed to drive the system could be self-produced for a modest cooling demand and up to 67% for the warmer conditions with high cooling demand.

Study on the effect of inclusion of thermal energy storage unit in the energy performance of a household refrigerator

Performance of two kinds of energy storage methods used in the household refrigeration is investigated and analyzed. A Mixture of De-ionized water with Natural graphite (3 wt%) and Paraffin (OM–46) were considered for phase change material (PCM) in the evaporator side and the condenser side respectively. After thorough investigation through experimental verification it is inferred that this set up has significantly enhanced the condenser heat transfer and the overall performance of the refrigerator. The variation in the compressor exit temperature exhibits a considerable dip in the maximum temperature of the compressor. This helps to increase the life time of the compressor. The evaporator and condenser temperature were approximately the same when the PCM is used on evaporator side. Under test conditions, this design of refrigerator could save energy by 16–22 % with PCM on condenser and 7–13 % with PCM on evaporator.

Development of a Miniature Loop Heat Pipe for High Heat Flux Devices and Evaluation of Its Heat Transport Performance

This paper describes the design, fabrication, and the heat transfer performance of a miniature loop heat pipe (mLHP) aiming for cooling a small and high heat flux devices. For the design process, a one-dimensional numerical model was developed. Based on the numerical model, an mLHP was fabricated. The mLHP comprises an evaporator (15×16×t3 mm3), a condenser (12×12.5×t2 mm3) and transport lines (O.D.=1.6 mm, L=100 mm). The mLHP uses Shirasu porous glass (SPG) as a wick and is mainly made of stainless steel. The heat transport performance was investigated in horizontal attitude, anti-gravity (top heat mode) and gravity-assisted (bottom heat mode) with working fluid of water. When the condenser was dissipated by natural air convection, the mLHP performed 5-W (20-W/cm2) heat transport at 170-℃ heat source temperature in horizontal attitude. The thermal resistance was 7.9℃/W. In the bottom heat mode, the lowest thermal resistance of 2.6℃/W was obtained due to the gravity assist. Additionally, the mLHP was tested under the condition that the evaporator side was kept horizontal but the condenser side was bent upwards by 90 degree around the middle of transport lines. Consequently, in low heat load condition the bent mLHP showed the same performance with the bottom heat mode, but shifted to the horizontal mode as the heat load increased. Thus, the heat transport performance was comprehensively investigated for considering various uses.

A phase-field method for boiling heat transfer

We present a phase field method for heat transfer in two-phase flow with boiling. The vapor/liquid interface evolution is modeled by the Cahn-Hilliard equation. The phase change rate is determined by accounting for the heat conduction balance on either vapor or liquid side of the interfacial area, depending on which side the temperature is assumed to be maintained at the saturation temperature during boiling. The velocity correction scheme proposed by Dong & Shen [27] is extended to solve the Navier-Stokes equations for a non-solenoidal velocity field, and the entropy viscosity method is employed for stabilization. The phase change model is verified by two-dimensional simulations of a vapor bubble growing in super-heated liquid and in film boiling. In both cases, mesh independence of the results is systematically performed. Subsequently, the method is applied to predict the growth of three-dimensional vapor bubble in a rectangular microchannel with boiling flow, achieving good agreement with experimental measurements and available simulation results using the level-set method. The numerical experiments demonstrate that the required mesh resolution for the phase field method is comparable with that of volume of fluid (VoF) and level-set methods.

Study on Evaporation Characteristics of Water in Annular Liquid Pool at Low Pressures.

In order to investigate the energy transfer mechanism and the nonequilibrium effect during water evaporation in its own pure vapor at low pressures, a series of precise measurements are conducted to obtain the temperature profile near the liquid–vapor interface and the evaporation rates in an annular pool in a closed chamber. The results show that the interface temperature of the vapor side is higher than that of the liquid side when water evaporates in its own pure vapor at low pressures (ranging from 394 to 1467 Pa), the temperature discontinuity across the interface exists in all experimental conditions. The magnitude of the temperature discontinuity is strongly affected by the vapor pressure. A uniform temperature layer with a thickness of about 2 mm is found below the evaporating interface because of the coupling effect of evaporation cooling and thermocapillary convection. The energy required for evaporation is mainly transferred by thermocapillary convection in the uniform temperature layer. Furthermore, the numerical simulation results confirm that the evaporation flux near the cylinders is much larger than that at the middle region, which implies that most of the latent heat required for evaporation is transferred to the interface near the cylinders.

Experimental investigation of a transcritical CO2 system for simultaneous heating and cooling performance

To analyze the refrigeration and heating performance of the transcritical CO2 system, an experimental study was carried out. The effect of the operating conditions on heating and cooling performance was investigated. The experimental results showed that the mass flow rate significantly increased with the compressor rotational speed, while the both heating coefficient of performance and refrigeration coefficient decreased with the compressor rotational speed. It was because the compressor power, cooling and heating capacity all increased with the compressor rotational speed, while the growth rate of compressor power was greater than that of cooling and heating capacity. Besides, the heating coefficient of performance became smaller in cold conditions, as the mass flow rate decreased because of the reduction in suction refrigerant density. Even so, when the heat source temperature in the evaporator side was equal to −18 ℃, not only the heating coefficient of performance is greater than 2.5, but also the system could provide enough heat. A new correlation of optimal discharge pressure was developed in this paper. Therefore, CO2 is suitable for replacing traditional refrigerants, and the experimental results can provide some guidance for designing a transcritical CO2 system.

Notes on the Phenomenological Model of Subcooled Liquid Boiling

It is shown that the Snyder–Bergles model seems to be the best phenomenological one while describing the process of forced boiling of the liquid considerably subcooled relative to the saturation temperature. This model most likely (without contradictions) accounts for the main physical subprocesses that accompany subcooled liquid boiling (liquid microlayer evaporation below the bubble, vapor condensation at the bubble dome, liquid inflow to the microlayer), meets modern concepts on the boiling mechanism, and agrees with the experimental data. Based on experiments at the Joint Institute for High Temperatures, Russian Academy of Sciences (JIHT RAS) using high-speed video recording (at a rate of up to 100 thousand frames per second) and other studies, some subprocceses incorporated into the model are refined. They are as follows: deactivation of the nucleation sites, heat removal from the bubble dome via unsteady heat conduction, and the bubble size evolution in time as the balance of masses of the evaporated and condensed vapor. It is shown that the main mechanism through which heat from the bubble dome is transferred to the surrounding liquid is unsteady heat conduction, the intensity of which is inversely proportional to the square root of the bubble existence time. The evolution of bubble sizes is caused by the balance of masses of the evaporating liquid microlayer and steam condensing on the bubble dome. With high heat flux densities supplied to the heating surface and low liquid subcooling values, the imbalance shifts toward the evaporation side, as a result of which conditions may emerge for a growth of large bubbles and their subsequent merging and occurrence of large vapor agglomerates in the coolant flow.

Heat recovery design and test for the secondary loop heat pump MAC system

Abstract In mobile air conditioning (MAC) systems, the secondary loop (SL) structure was usually adopted when using flammable refrigerants such as R1234yf, R290 and R152a. The introduction of intermediate heat exchangers in SL system inevitably increased the temperature difference between the condensation side and the evaporation side, thereby increased the compressor power consumption and caused the performance attenuation. To overcome the attenuation, this study designed a heat recovery device which collects the waste heat from the passenger compartment to increase the evaporation temperature and reduce the compressor power consumption. Validation experiments were completed in a secondary loop heat pump (SLHP) MAC system using R152a and was compared with a baseline direct expansion (DX) system. According to experimental results, under the -10 °C outdoor temperature, the SLHP system showed performance attenuation compared with the DX system. With the increase in indoor temperature, the attenuation gradually increased to 8.96% at the indoor temperature of 20 °C. In this case, a heat recovery design was used in the SLHP system, which increased the coefficient of performance (COP) by 9.29% compared with the original SLHP system and achieved similar performance to the DX system.

Investigation of performance and drying kinetics of the closed, partially open, and open heat pump drying systems

AbstractIn this study, the slices of “starking delicious” apple were dried at 40°C and an air velocity of 1 m/s in the heat pump drying system which was operated in three modes, that is, closed, open, and partially open. The model developed by Midilli and Kucuk was found to be the best model to explain the experimental data of apple slices drying. The effective moisture diffusivity values ranged from 2.33 × 10−8 to 3.06 × 10−8 m2/s. The evaporator and condenser side for open, partially open, and closed circuits are designed to operate with 100% fresh air or partial fresh air, which is controlled using dampers. Moisture content reduction of the apple slices from 4.631 to 0.126 g‐water/g‐dry matter moisture content took 404 min in the system operation with the condenser side partially open. Open heat pump drying system evaporator and condenser sides were completed in 27.4% less time than the open system application. The average performance coefficient (COPsys) of the entire system was found to be the highest in the closed system operation with 2.505. The SMERsys average was 80.80 g/kWh and the minimum total energy consumption was 5.580 kWh in the open heat pump drying system with the condenser side open.Practical ApplicationsIn this study, a heat pump dryer has been developed that can be operated in both closed, partially open and as well as open loop modes to achieve the best drying performance with the help of dampers placed on the heat pump drying system. The innovation of this study is that the system works more efficiently and the drying time is completed in a shorter time by utilizing the external condenser heat. Drying was performed in five different applications depending on the position of the dampers. Another purpose of this study was to fit the experimental data to the eight thin‐layer models and calculate the effective moisture diffusivity and activation energy. In addition, the color of the dried fruits was examined in terms of quality.