Refrigerant Heat Exchanger Research Articles

Energy, Exergy, and flow fields analysis to enhance performance potential of a heat-driven thermoacoustic refrigerator

This study focuses on the design, optimization, and analysis of a heat-driven thermoacoustic refrigerator system to be used in a medium-size Combined Heat and Power (CHP) system, using its exhaust hot gases. Two configurations are investigated to maximize the total Coefficient of Performance (COPTot). Several approaches, including energy, sensitivity, displacement, and exergy analyses were conducted to understand the underlying physics, identify more effective performance configurations, and to find areas with the most potential for improvements. In an energy analysis, the engine and resonator efficiency, and the refrigerator COP (COPRF) were examined. The effects of engine and refrigerator stacks and their heat exchanger lengths and spacings on engine efficiency and the COPTot were investigated. Work and heat transfer dynamics are investigated across the system. Component interactions were explored through studying the acoustic intensity and pressure and velocity phase difference (θPU) distribution, and variations in velocity and pressure amplitudes in the engine and refrigerator stacks. Displacement analysis was introduced to assess the impact of stack length variations on refrigerator and engine displacement amplitudes. The analysis revealed the significant influence of the hot heat exchanger in the engine (HHXe) and the cold heat exchanger in the refrigerator (CHXRF) on design optimality. Sensitivity analysis identifies that the performance indices of the refrigerator are mainly sensitive to the stack length and spacing. Additionally, based on three new performance indices, an exergy analysis identified why and how the refrigerator heat exchanger closest to the engine performs better.

Heat transfer process and anti-frost performance of finned evaporator with air–water dual heat source for building heat supply

ABSTRACT In this research, a dual heat source heat pump evaporator (AWSHP) with low-temperature hot water and air energy as the heat source is presented for the frosting problem of an air-source heat pump (ASHP) in low-temperature operating conditions. The critical frost region of the AWSHP system was also mapped using air temperature and water spray temperature as variables. The heat transfer characteristics as well as the anti-frost performance of AWSHP under different operating conditions were analyzed through experiments and simulations. According to the study's findings, the AWSHP system's refrigerant evaporation temperature was higher at the same ambient temperature than that of the ASHP. With AWSHP spray water volume of 8L/min and heat pump condenser circulating water volume of 15L/min, the average refrigerant heat exchanger and COP of AWSHP increased by 14.8% and 11.1%, respectively, compared with the ASHP system. At the same time, the AWSHP anti-frost operation strategy is proposed for low-temperature operating conditions. When the water spray volume was adjusted to 0.25 and 1.75 m³/h, respectively, under varying hourly meteorological conditions, the frost-free operation time was 61.21% and 89.24%, respectively. This was an improvement of 18.17% and 46.20%, respectively, compared to the ASHP.

Evaporation performance of propane in offset strip-fin plate heat exchanger for refrigerant-based battery thermal management in electric vehicles

This study investigates the potential of refrigerant-based cooling (RC) using a natural refrigerant (propane) and lightweight energy-efficient brazed plate heat exchangers (PHXs) in the battery thermal management systems (BTMSs) of the electric vehicles (EVs). Experimental assessments were conducted to analyze the evaporative heat transfer characteristics of propane within an offset strip-fin flow-structured PHX under various operational conditions resembling the BTMS operation. The battery discharge rates, often represented by the C-rate, are determined by the discharging current relative to the battery's capacity. In this study, a heat generation model based on the C-rate was employed to evaluate the thermal performance of the battery. Parameters, such as mass flux (G) at 30–70 kg/m2s, discharge rates (C-rate) of 1, 1.25, and 1.5C, and saturation temperatures (Tsat) of 16–24 °C were explored, with the inlet vapor quality (xin) in the range of 0.1–0.8. The influence of G and Tsat on the heat transfer performance showed that higher G values (60 and 70 kg/m2s) achieved the maximum heat transfer coefficients (HTC), which resulted in impractical pressure drops (PD). While higher HTC and lower PD were observed at a Tsat of 24 °C, concerns about increased inner wall temperature (Tiw) and safety emerged at this temperature. Hence, the optimum conditions of G (30 and 50 kg/m2s) and Tsat (16 and 20 °C) were investigated. The average HTC increased by 29, 27, and 21 % at 50 kg/m2s compared to that at 30 kg/m2s with Tsat of 20 °C, resulting in an average of 0.7, 0.9, and 1.2 °C reduction in the Tiw for C-rates of 1.5, 1.25, and 1C respectively. Compared to the Tsat of 16 °C, higher HTC and lower PD were observed at 20 °C, however, the Tiw was in the range of 21.12–20.47 °C for Tsat of 20 °C, which is 3.5–4 °C higher than that at Tsat of 16 °C. The results from the critical heat transfer coefficient analysis showed that the peak heat dissipation with minimal wall temperature was attained with optimal xm of 0.45–0.5, which enhanced the cooling efficiency for both the G values of 30 and 50 kg/m2s, and both the Tsat values (16 and 20 °C).

Design and optimization of ground‐coupled refrigeration heat exchanger in Dubai: Numerical approach

AbstractGround‐coupled heat exchangers (GCHE) have received significant attention over several decades as a result of increasing the world's energy demand and the need for reducing fossil fuels consumption. Prior studies have demonstrated the effectiveness of utilizing GCHE with refrigeration and heating systems. However, optimizing the performance of GCHE coupled with chillers for heat rejection, especially in extreme hot‐humid climates (where cooling towers are not very effective) is lacking in the literature. In this work, a ground borehole fitted with a coaxial‐tubes heat exchanger (BHE) is numerically simulated. Based on a wide range of data collected for the soil of Dubai, its real in situ thermophysical properties are characterized. The soil's upper layer thickness is relatively small and dry that operates in conduction mode, while the lower one is water‐saturated that works in coupled conduction‐advection mode. The study aims at optimizing the parameters advancing heat rejection into ground considering the actual properties of the soil of Dubai. The results indicate that the more feasible high‐density polyethylene pipes can perform as good as the steel ones. Also, a great finding based on the presented novel design is that insulating the inner pipe can increase the temperature duty by 55%. The proposed design of BHE is relatively inexpensive, more feasible and efficient, which is achieved for the first time based on a deep analysis of Dubai climate and soil. This makes the technology ready to be implemented for industrial applications in Dubai and other regions having a similar climate and soil nature.

Alloy Design for AM: A Ferritic Alloy for Applications in Corrosive Alkaline Environment

Alloy design allows to define specific compositions of materials having certain technological requirements as boundary conditions. Moreover increasing interest raised in the last decades on materials for Additive Manufacturing (AM). These new technologies consider a new approach, i.e. bottom up, for component manufacturing and suitable materials in different forms, generally liquid or solid (powders, filaments). One of the most interesting advantages of AM is the potentiality of realizing components with complex geometries and in a single or in few pieces to be assembled. For this reason, in this work a ferritic allow has been designed with the scope of realizing heat exchangers for highly corrosive alkaline environments. Water-ammonia solution are in-fact used in absorption machines for refrigeration and several heat exchangers are required for its thermodynamic cycle. After defining a specific composition, also on the base of thermodynamic simulations, the alloy has been produced by Vacuum Induction Melting (VIM). After suitable thermal treatments microstructural, mechanical and thermal characterizations have been carried out.

Review on Current Application of Microchannel Heat Exchanger

This paper considers various applications of heat exchangers in refrigerators, air conditioning, and nuclear power plants in recent years, current issues and trends, including the required capacity of refrigerator heat exchanger and the corresponding advantages of microchannel heat exchanger, refrigerator application of frost problems, ash problems and improve the performance of the relevant research; Examples of application of microchannel heat exchanger in air conditioning, comparison with traditional heat exchanger, and improvements to problems such as frost, the distribution of fluids, corrosion found in experiments; Study on advantages of compact microchannel heat exchanger in nuclear power plant. The purpose of this paper is to fill the gap in the application of microchannel heat exchanger and provide reference for future research.

Performance degradation of air source heat pumps under faulty conditions

The ongoing European regulations towards the decarbonization of energy intensive sectors such as heating and cooling will lead to an increase of heat pump appliances. Apart from anomalies that can be detected in a preliminary phase after the start-up, soft faults such as refrigerant leakage and heat exchanger fouling may cause a performance degradation in such systems that evolves in time and that must be detected before it becomes too big. From this perspective, in a previous paper (Pelella et al. [1]) it has been demonstrated that, in cooling mode, a seasonal performance penalization up to 50% can be reached in case of a not-planned maintenance scenario, whereas wherever a timed or intelligent maintenance strategy is implemented based on data-monitoring, this impact can be significantly reduced. Therefore, in order to extend the analysis previously carried out to the heating cases, and to evaluate the consequent energetic degradation of the system in case of soft faults occurring, depending on different climate conditions and maintenance scenarios, this paper develops a physic-based model to simulate system components and to describe the fault phenomenology on a residential 2.6 kW air-to-air heat pump, operating in winter mode. The results show that in heating mode a 40% condenser fouling and a 30% refrigerant leakage cause a performance degradation of respectively 16% and 12%, whereas in case of evaporator fouling the performance penalization is only of 3.2%. Moreover, the performance degradation is enhanced by the overlapping effect of simultaneous faults. Finally, from seasonal simulations of the heat pump along an entire machine lifetime of 12 years, it is found that none of the maintenance strategies analysed is able to significantly reduce the number of scenarios penalized by faults, opening to the potential development of systems for the automatic fault detection, diagnosis, and evaluation (FDDE).

Improving the performance of a commercial absorption cooling system by using ejector: A theoretical study

Absorption cooling systems (ACS) have lower coefficients of performance (COP) compared to direct expansion (DX) cooling systems. Nevertheless, ACS offers a green alternative to typical DX systems. In this study, a numerical model was developed for the commercial low-capacity Robur® absorption cooling system (RACS). The model was developed based on mass, concentration, and energy balance equations, in addition to heat transfer equations. The model results were validated against experimental data available in the literature for the same cooling unit yielding a good agreement. Hence, to improve the COP of the RACS, a vapor ejector was introduced between the generator and the condenser. An improvement of 70.6% in the COP was obtained at the design condition. A parametric analysis was implemented to study the significance of the key parameters in the RACS performance. It was found that the increase in the ambient temperature not only increased the activation temperature, but it also decreased the COP and increased the circulation ratio (CR). Consequently, in hot environments, lowering the evaporator temperature is recommended to avoid the need for higher CR. Optimizing the nozzle throat and the mixing tube diameter improves the ejector performance, and hence the RACS performance, as long as the ejector operates under critical conditions. Finally, the absorber coil was found to have the most significance on the RACS performance in comparison with the rectifier coil and the refrigerant heat exchanger.

Entropy Generation of Two-Phase Flow Boiling in Elliptical U-Bend Tube Heat Exchanger

In this research paper, the entropy generation was used as a criterion to measure the quality level of energy transfer for two-phase flow boiling of R134a in the elliptical U-bend tube. The eccentricities of the elliptical cross-section ranging from 0 to 0.904, were constrained to the same hydraulic diameter of 8 mm. The input parameters utilized in the numerical computations were mass velocities of 100 and 200 kg/m2 s for the inlet vapor qualities of 0.1 and 0.2 and a constant uniform heat flux of 10 kW/m2 imposed on the outer surface of the tube. The calculations were performed using the volume of fluid multiphase model coupled with the phase change model. The findings demonstrated that when the eccentricity of the U-bend tube increases, the quality of energy transfer in the heat exchanger improves as entropy generation reduces. The numerical calculations were validated and found to be consistent with what has been published in the open literature. The results of this paper are important for the development of heat exchangers for air conditioners and refrigerators.

Mechanically robust superhydrophobic copper surface with self-cleaning, anti-icing, and corrosion resistance

This paper uses a new method combining nanosecond laser processing technology and sol-gel to design and manufacture mechanically robust micro-nano structure and modified SiO2@PDMS coating on copper substrate. The superhydrophobic composite surface was successfully synthesized with a water contact angle of 159.5° and a sliding angle of 0.5°. The characterization of the prepared superhydrophobic surface #3 (laser-textured@SiO2@PDMS protective layer) was studied by optical profilometry, scanning electron microscopy, FT-IR spectroscopy, and X-ray photoelectron spectroscopy. The composite structure of micron/nano and organic/inorganic 3D interwoven network can be observed to improve the hydrophobic, wear-resistant, and compressive properties of the substrate surface. FT-IR spectra fully indicate the presence of PDMS and Si (CH3) grafted onto SiO2 particles on the copper surface, and the presence of SiO and SiC detected by XPS fully proves the FT-IR results. In addition, self-cleaning, wear resistance, anti-icing, corrosion resistance, and NaCl deliquescence behavior were evaluated. The results show that surface #3 can efficiently remove contaminants within 200 s. In the anti-icing experiment, the delayed icing time and ice adhesion strength of surface #3 are 3.33 times and 1.99 % of that of the ordinary copper surface, respectively, which can make the icicle slide easily. In the electrochemical test, the corrosion current density decreased by an order of magnitude, and the corrosion inhibition efficiency reached 90.57 %. Based on its excellent self-cleaning, mechanical robustness, anti-icing, and anti-corrosion properties, we believe that the composite modified surface of the laser-textured@SiO2@PDMS protective layer designed and manufactured in this study is expected to be used in engineering fields such as refrigeration heat exchangers.

Additive manufacturing of magnetocaloric (La,Ce)(Fe,Mn,Si)13–H particles via polymer-based composite filaments

Additive manufacturing could be an excellent way of shaping magnetocaloric heat exchangers in magnetic refrigerators. However, the metal additive manufacturing techniques present the serious limitation that the melting of a magnetocaloric material can cause its transformation and the loss of functionality. Fused deposition modeling using polymer-based composite filaments is presented as a promising alternative as temperatures are low enough to preserve the magnetocaloric material. To prove this claim, a polymer-based composite filament containing 55 wt% of (La,Ce)(Fe,Mn,Si)13–H magnetocaloric fillers has been manufactured using custom-made polymer capsules as the feedstock for the extrusion. Both adiabatic temperature change and isothermal entropy change have been characterized for the fillers, as-prepared filaments and as-printed parts, indicating that the magnetocaloric material functionality is not altered along the whole process. Printing resolution is comparable to the raw PLA filament.

An experimental study on the effect of vertical header geometry on the two-phase refrigerant distribution and performance of a microchannel heat exchanger

Micro-channel heat exchangers are widely used due to their superior performance compared to the existing fin and tube heat exchangers, but are limited to condensers due to refrigerant non-uniformity problems in evaporator operating conditions. Although many studies have been made to solve the refrigerant non-uniformity of the microchannel heat exchanger having a vertical header, a solution has not yet been proposed. In this study, a solution to the non-uniformity of the refrigerant was presented by applying a vertical header in the form of a double tube that can be applied to mass production. The effect of a double-tube form of the header on the refrigerant distribution and performance of a microchannel heat exchanger was explored by experimental means, using R-410A as the refrigerant. To investigate the effect of the shape of the header on the distribution of refrigerant in the final path of the heat exchanger and its performance, three types of multi-baffle and two types of multi-room header were considered. The tests were performed with an evaporation pressure in the microchannel heat exchanger of 800 kpa and a refrigerant flow rate of 50 to 80 kg/hr. The header with 5 independent refrigerant paths from the header inlet showed the most uniform distribution. The results showed a linear relationship between refrigerant distribution and heat exchanger performance. This study showed the effect of improving heat exchanger performance by up to 12% with the improved refrigerant distribution by applying a vertical header in the form of a double tube.

Energy efficient large-scale storage of liquid hydrogen

The world’s largest liquid hydrogen storage tanks were constructed in the mid-1960s at the NASA Kennedy Space Center. These two vacuum-jacketed, perlite powder insulated tanks, still in service today, have 3,200 m3 of useable capacity. In 2018, construction began on an additional storage tank at Launch Complex 39B. This new tank will give an additional storage capacity of 4,700 m3 for a total on-site storage capacity of roughly 8,000 m3. NASA’s new Space Launch System (SLS) heavy lift rocket for the Artemis program includes an LH2 tank that makes up the bulk of the vehicle, holding 2,033 m3 of LH2 in its 8.4-m diameter by 40-m height. The new storage tank includes two new energy-efficient technologies: a glass bubbles insulation system in lieu of perlite, and an Integrated Refrigeration and Storage (IRAS) heat exchanger for controlled storage capability. The evacuated glass bubbles insulation system is based on the prior two decades of research to prove the thermal performance benefits as well as the mechanical and vacuum integrity; and has been shown to reduce LH2 boiloff by 46% versus perlite in field demonstrations. The IRAS capability is centered on a heat exchanger system that is built within the inner vessel to reject heat from the bulk liquid through the future implementation of an external helium refrigerator. Controlled storage via IRAS, when fully implemented, will provide full control of the ullage pressure, zero boiloff, and even production of densified LH2 pending its adoption on future launch vehicles. The design basics are described along with main construction and testing processes involved. The key features of the new technology items and implications on simplified operations and long-term energy savings are addressed.

A research on silver powder sinters for dilution refrigerator heat exchangers

Due to the Kapitza resistivity, a sharp deterioration of heat transfer occurs between solid and liquid in the heat exchanger of the dilution refrigerator at extremely low temperature. It is necessary to use silver powder sintered heat exchangers to optimize the interface heat transfer. A theoretical calculation of heat exchangers at extremely low temperature was carried out to analyze the influence of the Kapitza resistivity on heat transfer performance. Silver powder with different particle sizes of 80 nm, 200 nm and 500 nm were selected for the preparation of sinters. Their micro-scale sintering conditions, pore volumes and specific surface area were carefully presented. It can be concluded from the theoretical calculations and the experimental results that the 200 nm silver powder sinters can optimize the performance of dilution refrigerator heat exchanger with a large heat exchange area.

Simultaneous synthesis and optimization of refrigeration cycles and heat exchangers networks

• Refrigeration cycle structure, pressures/temperatures and HEN are optimized. • An ad hoc decomposition algorithm for the problem solution is devised. • Results indicate considerable cost (−40%) and power consumption (−52%) • Major advantages found for case studies with multiple hot and cold streams. • Even if the problem size is large, the required computational time is practicable. This work proposes a simultaneous approach for the synthesis and design optimization of industrial refrigeration cycles integrated with heat exchanger networks. Given the process hot and cold streams, the methodology determines the economically optimal refrigeration cycle configuration (number of evaporation/condensation levels, compressor intercooling, multiple throttling, etc.),cycle variables and heat exchanger network. The methodology includes a novel refrigeration cycle superstructure capable of reproducing a wide range of cycle architectures and an effective solution algorithm (based on the decomposition of the problem on two levels) to tackle the challenging Mixed Integer Non-Linear Program. The application to four literature case studies indicates that the proposed approach returns solutions which are considerably better both in terms of efficiency and economics than those published in literature. The application to four literature case studies indicate that the required computation time is tractable even for problems with thousands of variables and constraints (up to 87,000 real variables, 29,000 binary variables and 129,000 equations). Moreover, compared to previous literature studies, the optimized solutions feature a total annual cost reduction up to 40% and a decrease in compression power consumption up to 52%.

Combined effects of refrigerant leakages and fouling on air-source heat pump performances in cooling mode

• Digital model of an electric air-to-air heat pump for domestic cooling. • Fault Detection and Diagnosis tools for the identification of operating anomalies. • Combined refrigerant leakages and heat exchanger fouling soft-faults analyzed. • Impact of faults on thermodynamic cycle parameters and COP. • Seasonal performances and TEWI for different climates and maintenance strategies. For electric heat pump systems (EHP), soft faults such as refrigerant leakages and heat exchangers fouling can lead to significant performance degradations, which can remain unidentified for a long period. This paper, through a digital model, wants to simulate the cooling season behavior of a case study air-source heat pump for residential air-conditioning, analyzing the performance degradation in case of both standalone and simultaneous occurrence of soft faults. The study is carried-out in different scenarios of not planned and ordinary maintenance of the machine and in typical climate conditions of Naples, Miami and Shanghai. The impact of faults on seasonal performances (SCOP) and total equivalent warming impact (TEWI) has also been analyzed. Results show that refrigerant leakages can have an impact on system performance higher than 25%, whereas heat exchangers fouling can downgrade the efficiency of approximately 15%. It is also found that, for each climate condition, a different maintenance strategy can halve the cases with SCOP penalization higher than 10%. Since faults have not the same probability of occurrence, a smart monitoring and a fault detection system can be the solution to guarantee good performances avoiding at the same time excessive maintenance costs.

Investigation on the performance enhancement of electric vehicle thermal management system utilizing floating loop with finite heat exchanger size

This study suggests an electric vehicle heat pump and thermal management system employing a floating loop. The floating loop manages the thermal state of power electronics and electric motors by utilizing the liquid refrigerant at the condenser outlet. This loop recovers the waste heat generated by power electronics and electric motors in winter and enhances the cooling performance in summer through the superior cooling performance of evaporative heat transfer. Configurations of heat pump and thermal management system are presented with operating schematic in summer and winter. To verify the performance of the suggested system, the heat pump and thermal management system model is established based on experimentally validated heat pump component models and electric device models. Heat exchangers are reallocated within a finite volume to investigate the effect of thermal load shift from the coolant to the refrigerant. The result shows that the heat pump system utilizing a floating loop with the largest refrigerant heat exchanger can save power consumptions in winter up to 27.7% and 5.8% in summer while maintaining the thermal state of electric devices within the appropriate range.

Exergy destruction rate minimization in the absorber of a double effect vapor absorption system

Despite the wide applications of multi-effect vapor absorption systems, their energy requirement is relatively higher. Also, their exergy analyses found in the literature reveal that the exergy destruction rate at the absorber is quite significant and has the potential for improvement in its energy efficiency. In this work, the exergy destruction rate at the absorber is minimized using the penalty factor method against the optimized generator temperature of the double-effect vapor absorption system by considering absorber, evaporator, and condenser temperatures into consideration. Modeling of the double-effect vapor absorption system was performed using a thermodynamic toolbox in SIMULINK. The present model employed a refrigerant heat exchanger to enhance the system cooling capacity. The liquid-vapor ejector valve at the absorber also improved the mixing of the solution and refrigerant vapor resulting in lower irreversibility of the system. Results show that the coefficient of a performance increase by 2.4% with refrigerant heat exchanger and exergy loss at absorber decrease by 9.4% with ejector. The optimum performance was seen at the condenser and evaporator temperatures of 308.8 K and 278.1 K, respectively with an 8.2% improvement in exergetic efficiency. Finally, it is concluded that the multi-effect absorption system shows better performance by minimizing the irreversibility.

Proposed new equations for calculation of thermophysical properties of nanofluids

A trial-error procedure is applied for the derivation of correlations to estimate the relative thermal conductivity (kr) and dynamic viscosity (µr) of nanofluids using MATLAB. Thermophysical properties of particles and base fluids, particle diameter (dp), sphericity, capping layer thickness, Brownian motion of a particle, temperature, and volume fraction (φ) are considered. The accuracy of predicting kr and µr of nanofluids is developed using dimensionless parameters involving base fluid and particle characteristics. The results reveal that the estimated values are in a good agreement with the experimental data with a standard deviation of 2.16% and 8.16% for kr and µr of nanofluids, respectively. Besides that, 97.5% of the predicted kr values suit experimental data of kr with a mean deviation of ±5%, whereas 90.4% of the estimated µr values match the data of µr with a mean deviation of ±10%. Therefore, the proposed new equations will be useful for numerical simulation studies and the engineering design of heat transfer devices such as refrigeration systems, solar collectors, and heat exchangers.

Transient behavior assessments of a single-effect ammonia-water absorption heat pump system: Development of an efficient experimentally validated numerical framework

In this study, the long-term transient response of a single-effect ammonia-water gas-fired absorption heat pump prototype is investigated both numerically and experimentally. For the numerical model, heat and mass transfer components are divided into four groups; (a) tubular heat exchangers comprising condenser, evaporator, absorber, solution and refrigerant heat exchangers, (b) tray column heat exchanger comprising gas-fired generator, (c) refrigerant and solution tanks, (d) solution pump and restrictors. Components of groups a-c are modelled based on discretized volumes in which the unsteady mass, species and energy conservation equations are imposed, whereas solution pump and restrictors are idealized as algebraic models. The experimental analysis covers a period of over 10 h, during which various step changes are applied to system inputs like gas power, inlet water temperature, solution and refrigerant flow rates, and brine inlet temperature. The results of the numerical model are compared with the experimental data which show good agreement in both transient and near steady state operation. Results show that, during transient operation average deviations are less than 2.94% and near steady-state conditions are less than 1.70%. The accuracy of the numerical model in predicting both the transient response and the steady-state values of the measurable outputs, makes it a valuable tool to gain insight on the internal dynamics of ammonia-water absorption heat pumps and cut down costs on time-consuming optimization experiments, such as the fine-tuning of initial solution charge, the implementation of a suitable concentration control strategy and the performance optimization at full and partial loads.