Refrigeration Cycle Research Articles

Analysis of HEM applicability for a subcritical flashing ejector at low motive pressure

Validated CO2 ejector models are essential for developing high-performance refrigeration and heat pump cycles. This study focuses on assessing the Homogeneous Equilibrium Model’s applicability to simulate a CO2 flashing ejector at a reduced pressure of 0.47. The model was implemented in FLUENT, integrating a user-defined real gas model. Simulation results with different boundary condition options were compared to experimental data. The analysis was carried out to evaluate the predictive capabilities of the model and assess the experimental data quality. The results indicate that the developed model accurately estimated the motive mass flow rate, with a maximum relative error of 5.7%, showing better performance than previously reported data. The entrained flow rate, assuming double choking operation, was significantly higher than the experimental measurement, and the CFD-predicted wall static pressure underestimated the experimental profile, suggesting single-choked ejector operation. In contrast, the outflow density was better predicted under the same assumption, with an average error of 8.6%. Nevertheless, the simulated temperature profiles showed good agreement with the experimental data, especially when using the experimental entrained mass flow rate as a boundary condition.

Numerical Investigation of Two-Phase Shock Waves in CO2 Flows Using a Modified Hertz–Knudsen Model

Abstract Understanding the complex behavior of two-phase shocks in CO2 flows is essential for a variety of applications, including carbon capture and storage () and transcritical refrigeration cycles. This study presents a comprehensive numerical investigation of two-phase shock waves using the multispecies user-defined real gas model in Ansys Fluent. The simulations are performed for de-Laval nozzles, exploring the two-phase shock features for three-dimensional (3D), two-dimensional (2D), and two-dimensional axisymmetric geometries. The nonequilibrium condensation, subsequent evaporation, and denucleation occurring across the shock are modeled through a set of user-defined scalar transport equations implemented within Ansys Fluent. The two-phase computational fluid dynamics (CFD) simulations are carried out in proximity to the critical point where real gas effects are relevant. The CO2 real gas properties are computed using an in-house Python code and integrated into the solver via user-defined functions as external look-up tables. This study provides valuable insights into the physical processes underlying two-phase shocks in CO2 flows and their sensitivity to geometric variations and thermodynamic conditions. The findings contribute to the development and modification of predictive models and optimized designs for systems involving two-phase CO2 flows. The results highlight the influence of geometry configurations and thermodynamic conditions on shock location and intensity, providing comparisons for shock waves occurring in two-phase flows and supercritical single-phase flows.

Anomalous electrocaloric behaviors in (anti)ferroelectrics: a mini-review

Solid-state cooling, represented by the electrocaloric effect (ECE) in (anti)ferroelectric materials, has emerged as an alternative green refrigeration technology by virtue of its high efficiency and miniaturization and is expected to substitute conventional vapor-compression. Significant progress has been made in developing high-performance EC materials since its revival. However, anomalous EC behaviors are frequently observed, including asymmetric and negative EC profiles, and the physical mechanism behind this is still under debate. Its rationalization is of great importance since full utilization of anomalous EC behaviors could enhance EC strength and/or cooling capacity. This mini-review gives a brief overview of research advances in EC anomalies in (anti)ferroelectrics with the hope of provoking thought on the design of reconstructed refrigeration cycles and superior EC materials for application in solid-state cooling devices.

Enhancing Semiconductor Chiller Performance: Investigating the Performance Characteristics of Ultra-Low-Temperature Chillers Applying a Liquid Receiver

This study investigates the implementation of a cryogenic chiller utilizing a mixed-refrigerant cascade refrigeration cycle (MRCRC). In this setup, R-404A is employed in the high-temperature circuit (HTC), while a mixture of refrigerants is utilized in the low-temperature circuit (LTC). Unlike a conventional MRCRC that operates without a receiver to maintain the composition ratio, this research explores the impact of receiver installation on system performance. Experiments were conducted with and without a receiver to assess performance improvements and device behavior. With a fixed refrigerant charge of 4 kg, the suction and discharge pressures of the LTC compressor remained low and stable after the receiver’s installation. The addition of a receiver significantly reduced the cooling time, with further reductions observed as the refrigerant charge increased. The system achieved evaporative heat capacities of 0.59, 1.76, and 2 kW for refrigerant charges of 4, 7, and 9 kg, respectively. Notably, at the maximum refrigerant charge of 11 kg, the evaporative heat capacity peaked at 3.3 kW. These findings indicate that incorporating a receiver is crucial for enhancing the cooling performance of cryogenic coolers using mixed refrigerants and stabilizing device operation. This contrasts with previous studies that omitted receivers due to concerns over potential alterations in the composition ratio of the mixed refrigerant.

Theoretical performance study on R600/R170 auto-cascade refrigeration cycle with mechanical pre-cooling

A mechanical pre-cooling auto-cascade refrigeration (PACR) system was proposed, R600a single-stage vapor compression system was used to provide pre-cooling for R600/R170 auto-cascade refrigeration (ACR) system. Performance of PACR system under different operating conditions was discussed. The result showed that maintaining a constant outlet temperature of 30°C for the condenser and −45°C for the evaporator in a PACR system resulted in an optimal intermediate temperature that maximized coefficient of cooling performance (COP). At this time, the optimal intermediate temperature of R600/R170 was 0°C, the COP maximum value ( COPmax) was 0.693 and exergy efficiency ( ηEx) was 20.9%. And as the evaporator outlet temperature decreases to −70°C, the optimal intermediate temperature decreases to −10°C. The cooling capacity ( Qe) of the low-temperature grade circulating refrigerant in the PACR system using R600/R170 at different evaporation temperatures was increased by 86.3%–108% compared to that of R134a/R23. COPmax was increased by 7.1% compared to that of R134a/R23 at the evaporator outlet temperature of −60°C.

Augmentation strategy toward superior water absorption performance: An attempt utilizing [EMIm]Ac ionic liquid/water pair

Ionic liquids (ILs) offer significant advantages as alternative absorbents in absorption refrigeration cycles due to their superior physicochemical properties. Assessing the performance of new IL-based working pairs in absorption systems is essential for their rapid application in industrial applications. This study evaluates the water absorption performance of an aqueous solution of 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) under both saturated atmospheric and vacuum conditions. Additionally, the performance of an absorption chiller utilizing the [EMIM]Ac/H2O working pair was analysed using the Aspen Plus process simulator. A discrepancy was observed between the water vapor pressure in the air and the saturated vapor pressure of the aqueous solution at equilibrium. A theoretical criterion for the minimum relative humidity required for effective water vapor absorption by this absorbent at a given mass fraction is proposed. Under vacuum conditions, lower absolute pressure or higher mass fraction enhances absorption. Surfactants significantly improve absorption performance, with 2-octanol and 1-octanol demonstrating approximately double the absorption capacity compared to [EMIM]Ac. Higher mass fractions of the aqueous solution enhance the absorption rate but reduce the desorption rate. The coefficient of performance (COP) of the system decreases with increasing pressure and mass fraction, reaching a peak of 0.9 at a mass fraction of 20 % and a pressure of 3 kPa. Lower pressure and mass fraction enhance exergy efficiency.

Device Model for a Solid‐State Barocaloric Refrigerator

Solid‐state refrigeration represents a promising alternative to vapor compression cooling systems. Solid‐state devices based on magnetocaloric, electrocaloric, and elastocaloric effects have demonstrated the ability to achieve high‐efficiency, reliable, and environment‐friendly refrigeration. Cooling devices based on the barocaloric (BC) effect—entropy change due to applied hydrostatic pressure, however, has not yet been realized despite the significant promise shown in material‐level studies. As a step toward demonstrating a practical cooling system, this work presents a thermodynamic and heat transfer model for a BC refrigerator The model simulates transient thermal transport within the solid refrigerant and heat exchange with hot and cold thermal reservoirs during reversed Brayton refrigeration cycle operation. The model is used to evaluate the specific cooling power (SCP) and coefficient of performance (COP) of the device comprising nitrile butadiene rubber (NBR) as a representative BC refrigerant. Experimentally validated BC properties of NBR are used to quantify the contribution of different operating parameters including cycle frequency, applied pressure, operating temperatures, and heat transfer coefficient. The results show that a BC refrigerator operating with a temperature span of 2.4 K and 0.1 GPa applied pressure can achieve an SCP of 0.024 W g−1 at 10 mHz cycle frequency and a COP as high as 5.5 at 1 mHz cycle frequency—exceeding that of conventional vapor compression refrigerators. In addition, to identify key refrigerant properties, the effect of bulk modulus, thermal expansion coefficient, heat capacity, and thermal conductivity on device performance are quantified. The results highlight the trade‐off between different material properties to maximize the BC response, while minimizing mechanical work and improving thermal transport. This work demonstrates the promise of solid‐state cooling devices based on soft BC materials and provides a framework to quantify its performance at the device‐level.

Performance improvement and multi-objective optimization of a two-stage and dual-temperature ejector auto-cascade refrigeration cycle driven by the waste heat

This paper presents a two-stage and dual-temperature ejector auto-cascade refrigeration cycle (TEARC) driven by the waste hot water and low-pressure exhaust steam in chemical plants. In this cycle, a throttle valve is provided between the condenser outlet and separator inlet to regulate the composition and mass flow ratio of mixture refrigerant in the two evaporators. The specific enthalpies of refrigerant at the low-temperature (LT) evaporator and medium-temperature (MT) evaporator inlet are respectively reduced via the evaporative condenser and separator, leading to an enhancement in cooling capacity. Energy, exergy, and economic (3E) analyses are conducted to compare the performance between the TEARC and the basic two-stage and dual-temperature ejector refrigeration cycle (BTERC). At the basic operating condition, the TEARC has enhancements of 9.13 % and 9.95 % in COP and exergy efficiency than those of the BTERC. According to the multi-objective optimization results, it indicates that the TEARC's COP, exergy efficiency, and levelized cost of cooling are respectively improved by 8.93 % and 5.67 % and reduced by 1.22 % compared to the BTERC at optimum operating conditions. The simulation results of the proposed cycle reveal a significant performance improvement and the potential for application in waste heat-driven refrigeration.

Analyze the effect of external conditions of the cooling cycle on precooled linde systeme

This study investigates the influence of critical operating variables on the efficiency of the precooled Linde cycle, with a focus on refrigerant mass flow rates, condensation temperatures, and cooling power. The study explored condensation temperatures ranging from 2°C to 45°C and refrigerant mass flow rates between 0.1 and 0.9 g/s to understand their impact on system performance. Three refrigerant mixtures R229, R134a, and R410a were analyzed to evaluate their effect on key performance metrics, including the figure of merit and the proportion of gas liquefied. Nitrogen was utilized as the standard permanent gas to benchmark the system's overall effectiveness. The results demonstrate that lowering condensation temperatures enhances the system's efficiency, resulting in increased liquid yield, especially at higher refrigerant mass flow rates. The characteristics of the refrigerant used in the cold circuit emerged as a crucial factor influencing the efficiency of Linde’s precooling cycle. This research offers valuable insights into optimizing operating parameters, contributing to the development of more efficient precooled Linde systems. By improving system performance, this study advances the understanding of cryogenic refrigeration cycles, providing a pathway for future enhancements in design and application.

Optimizing building energy efficiency with a combined cooling, heating, and power (CCHP) system driven by boiler waste heat recovery

In light of increasing challenges related to fossil fuel consumption, global warming, and rising energy costs, optimizing energy systems has become essential. This study introduces a multi-purpose system designed to capture and utilize waste heat from boiler flue gases at the Tous power plant for simultaneous power generation, cooling, and heating. The system integrates an Organic Rankine Cycle (ORC) for power generation and an Absorption Refrigeration Cycle (ARC) for cooling, with the added function of recovering waste heat to preheat natural gas for the boiler and power plant. The system's performance was rigorously analyzed through energy, exergy, and exergoeconomic assessments. To determine the optimal operating conditions, a multi-objective optimization was conducted using the TOPSIS technique, focusing on critical parameters such as flue gas exit temperature, natural gas inlet temperature, and turbine inlet pressures. The results identified optimal conditions with a flue gas exit temperature of 532.5 K, a natural gas inlet temperature of 285 K, and turbine inlet pressures of 2.54 MPa and 14.62 MPa for the first and second turbines, respectively. Under these conditions, the system achieved an exergy efficiency of 41.1 %, cooling capacity of 133.1 kW, power generation of 210.4 kW, heat transfer to natural gas of 979.6 kW, and operational costs of 8.53 $/h. This study highlights the potential of the proposed system to significantly enhance energy efficiency and reduce operational costs in practical applications, offering a sustainable solution for energy management.

Efficient ecological function analysis and multi-objective optimizations for an endoreversible simple air refrigerator cycle

Abstract Combining finite time thermodynamics and exergetic analysis, analogous to the definition of ecological efficient power for heat engines, this paper proposes a unified performance indicator for various cycles, exergy-based efficient ecological function (E ɛ ) which is defined as product of exergy-based ecological function and coefficient of performance, and introduces it into performance optimization of endoreversible simple air refrigerator cycle coupled to constant-temperature heat reservoirs. Relations among E ɛ , pressure ratio (π) and heat conductance distribution ratio (u) are derived by using numerical method. The cycle performance indicators which include cooling load (R), coefficient of performance (ɛ), and exergetic loss rate (E out/T 0) under the different maximum objective criteria are compared. Taking π as optimal variable, and taking R, ɛ, cooling load density (r), E ɛ and their combinations as optimization objectives, multi-objective optimizations, totally 15 optimization combinations, are performed by using NASG-II algorithm. The results demonstrate that, the maximum E ɛ criteria can better reflect the compromise among R, ɛ and E out/T 0. The Pareto solution sets are majorly distributed in 2.5–20 when quadru-objective optimizations are performed. The option selected by LINMAP decision-making method is closer to ideal solution when bi-objective optimization of ɛ and r is carried out.

Development of a geothermal-driven multi-output scheme for electricity, cooling, and hydrogen production: Techno-economic assessment and genetic algorithm-based optimization

This study aims to develop an environmentally friendly multi-energy system for sustainable production of electricity, cooling and hydrogen. The study introduces a pioneering geothermal-based system, integrating an ejector refrigeration cycle, a dual-loop organic Rankine cycle, and a hydrogen production unit with proton exchange membrane electrolyzers. The study provides a thorough analysis of the system's energy and exergy performance, as well as its economic feasibility. Through sensitivity and parametric analyses, the research identifies key parameters that significantly influence system performance. The system's innovative design promises minimal environmental impact while delivering multifaceted performance: generating 1.38 MW of electricity, supplying 436 kW of cooling load, and producing 5.39 kg/h of hydrogen. In the exergy analysis, Evaporator1 is identified as the primary contributor to exergy loss, representing 34 % of the total exergy destruction. This is followed by the electrolysis unit, the condenser, and the ejector refrigeration cycle, which contribute 18 %, 14 %, and 12 %, respectively. The system achieves optimal efficiency at an organic Rankine cycle turbine1 inlet temperature of 387 K, yielding a power generation of 885.4 kW and an exergy efficiency of 26.7 %. Beyond this temperature, any further increase leads to a decline in power output due to operational disturbances. A multi-criteria optimization using genetic algorithm is applied, resulting in an optimized system with a cost rate of 18.13 $/h and an exergy efficiency of 38.96 %.

Analysis of the exergy transfers in subcritical and transcritical CO[formula omitted] ejectors and their connection with the performance of the refrigeration cycle

Ejector refrigeration systems (ERSs) operating with carbon dioxide are notoriously challenging from both design and operation perspectives. In particular, the ejector itself gives rise to special attention since it directly affects the performance of the global system. In this study, the connections between local exergy transfers within the ejector and global cycle performance are investigated by using a novel multi-scale numerical approach. Computational Fluid Dynamics (CFD) is used to generate accurate predictions of the ejector operation with access to the local flow and heat transfer features. The exergy tube analysis is applied onto the simulation results, linking local transfers within the ejector to the results at the component- and cycle-scale. A state-of-the-art ejector thermodynamic model is then calibrated onto the CFD results, and employed to determine the physically possible operation of the ERS. In addition, the metastability impact on the exergy transfers and overall system performance is investigated by comparing the CFD results of the classical Homogeneous Equilibrium Model with those obtained with the Homogeneous Relaxation Model. Notably, it is found that at fixed ejector inlet conditions, there exits an outlet pressure that maximizes the exergy transfers from primary to secondary streams. Interestingly, this work shows for the first time that the maximum exergy gain of the secondary stream appears to correlate with the maximum coefficient of performance of the system, independently of the on- or off-design operation of the ejector. Moreover, metastability generally deteriorates the performance at both scales. The results of the present study serve to pave the way towards better ejector design objectives.

A statistical physics-based methodology for evaluating the efficiency of an adsorption refrigeration cycle using C3H2F4/Maxsorb III

Recently, the statistical physics modeling found a great success in the description of various aspects of adsorption phenomena such as olfaction, depollution, and photovoltaic processes. Therefore, we attempted here to apply this treatment in the domain of refrigeration by an adsorption/ desorption process cycle, using the grand canonical formalism of the statistical physics as a working tool. So, for this study, four advanced equations based on statistical physics treatment are used to clarify the adsorption process at molecular level and define its physico-chemical parameters for refrigeration applications. Our results showed that the double layer model with two energies provides useful details concerning this phenomenon by determining the number of captured HFO-1234ze (E) (C3H2F4) molecules per site nAM, the density of receptor site Drs, the adsorption quantity at saturation N a sat and two energetic parameters Phs1 and Phs2. The steric results revealed that the adsorbed C3H2F4 molecules tended towards a parallel position when the temperature increased. Evaluations of the adsorption energies and enthalpy values indicated that the anchoring process is exothermic and of a physisorption nature. In order to improve the thermodynamic efficiency, the coefficient of performance was calculated by analyzing changes in thermodynamic functions, giving a value of 0.58 for the C3H2F4 / Maxsorb III. Also, the COP value can be enhanced by using the same adsorbate with high compressibility to minimize the machine size but changing the present adsorbent by activated carbon derived from biomass such as: Waste Palm Trunk (WPT-AC) and Mangrove wood (M−AC).

Optimization and exergoeconomic analysis of a biomass-driven cogeneration system for power and cooling applications

Due to the increasing population and the expansion of industry in daily life, a significant depletion of fossil fuel resources is observed, accompanied by escalating environmental concerns. In response to these challenges, this study presents an optimized cogeneration system powered by renewable biomass, designed for simultaneous power and cooling generation. The system integrates a biomass gasification unit, a steam Rankine cycle, a helium-driven gas turbine cycle, and an ammonia-water-based two-stage absorption refrigeration cycle with a bleed line. Thermodynamic and exergoeconomic analyses reveal a net power output of 5356 kW, a cooling load of 360.9 kW, and an exergetic efficiency of 29.52 %. Financially, the system shows a net present value of 11.76 $M and a payback period of 6.51 years. A multi-objective optimization utilizing the gray wolf algorithm improves system performance through the employment of three diverse scenarios, which raises power output to 5864 kW, cooling load to 372 kW, and optimize net present to 14.28 $M with a reduced payback period of 5.96 years. The studied system is well-suited for industrial applications, rural electrification, and district energy systems, and offers a sustainable and economically viable energy solution.

Comprehensive performance analysis and optimization of an ORC-HDH system for power and freshwater production driven by marine exhaust gas

Freshwater serves as a vital energy source for ships, underscoring the paramount importance of research into desalination systems in facilitating diverse and reliable energy supply for maritime vessels. Aiming at numerous waste heat of marine engines and the shortage of freshwater resources on board, this paper designs an innovative waste heat recovery (WHR) system. It includes an organic Rankine cycle, a compression refrigeration cycle, and a humidification-dehumidification (HDH) desalination system to recover waste heat from exhaust gases in cascades, which can effectively produce power, cooling, and freshwater, respectively. A parametric study of the co-generation system in terms of the freshwater production, gained output ratio (GOR), cost per unit of freshwater production, net power output, and per unit of output power cost, are conducted to identify the key operating parameters. A set of operating parameters is determined for the HDH system with a temperature of the humidifier outlet of 358.15 K and a mass flow rate ratio (MR) of seawater to humid air of 4.5. The effect of the relative humidity and temperature of the humid air, as well as the salinity and temperature of the seawater, on the performance of the HDH subsystem are analyzed. And the optimal operating conditions of the co-generation system are obtained through multi-objective cuckoo search optimization. The results demonstrate that the co-generation system can produce an output-power of 1479.8 kW, 500 kW, and 0.49 kg/s for cooling and freshwater production by using the working-fluid R365mfc/Cyclopentane with a mixing ratio of 33:67 under optimal conditions. The GOR and cost per unit of HDH subsystem are 1.62 and 1.70 $/m3, with a capital payback period of 4.18 years. The equivalent output-power of the co-generation system is 9.5 % higher than that of an ORC system.

Multi-criteria optimization next to a comparative analysis of a polygeneration layout based on a double-stage gas turbine cycle fueled by biomass and hydrogen

The significance and advantages of utilizing renewable energy sources are widely recognized. Upon examining past studies, it was revealed that there has been a lack of research on harnessing the waste energy from double-stage gas turbines powered by biomass-hydrogen hybrid fuel for polygeneration applications. Thus, a new configuration is proposed in the study for the multi-production of electricity, cooling, heating, and potable water. In the current research, a high-pressure turbine is driven by a combustion chamber fed by a biomass gasifier. Meanwhile, a low-pressure turbine is driven by a post-combustion chamber fed by hydrogen as fuel. The heat loss of the topping gas turbine cycle is recuperated by a supercritical Brayton cycle (SBC), a steam Rankine cycle (SRC), and a water heater. The waste heat of the supercritical Brayton cycle and steam Rankine cycle is recovered through an absorption refrigeration cycle and a thermoelectric generator, respectively. Thus, the heat loss of the topping system is completely recovered. The net power output of the gas turbine cycle is considered the electricity production of the system while the output power of the SBC and SRC is utilized in a reverse osmosis desalination system to produce potable water. The investigations conducted on the system are thermodynamic and exergy-economic assessments and three-objective optimization utilizing 5 gases (i.e., CO2, CO, Nitrogen, Air, and Helium) in the SBC. The final results reveal the superiority of Helium as the supercritical gas, bringing about an exergy efficiency () of 32.11%, a total cost rate () of 103 , a payback period (PP) of 0.159 years, and a specific cost of polygeneration () of 22.03 for the system. Although the levelized cost of products of the configuration is high due to the freshwater production, of the proposed system is higher than the previous systems based on a double-stage gas turbine cycle driven by biomass and hydrogen.

4E analysis of novel waste heat-driven combined cooling and power (CCP) systems based on modified Kalina-ejector refrigeration cycles

Global warming and environmental pollution are becoming increasingly severe problems due to the excessive use of fossil fuels. Recovering and utilizing waste heat from different energy systems can considered a sustainable solution to this destructive trend. This study proposes two modified Kalina cycle structures that use ejector refrigeration for combined cooling and power (CCP) generation in low-temperature waste heat. While to recover energy from a lean ammonia solution uses a two-phase expander. Thermodynamic analysis shows that the modified Kalina cycles E and F operate with a thermal efficiency of 0.3066 and 0.2557, respectively, representing a significant improvement of more than 123% and 86%, respectively, compared to the base cycle. Also, the exergy efficiency of the improved configurations E and F has been calculated as 0.083 and 0.089, respectively, showing an improvement of 53.8% and 64.8%, respectively, compared to the base cycle. The exergoeconomic analysis also revealed that the total capital cost rate for improved cycles E and F is estimated at 2.62 and 2.76 $/h, respectively. In addition, from an environmental perspective, the improved Kalina cycle F has proven to have a more eco-friendly performance with a pollution index of 0.2281 mPts/s. Moreover, a parametric study is conducted to investigate and analyze the effect of changes in significant performance parameters such as ammonia concentration, evaporator temperature, turbine inlet temperature and pressure, and turbine back-pressure for both presented modified cycles.

PRACTICAL SESSION USED FOR A DEEPER UNDERSTANDING OF VAPOUR COMPRESSION SYSTEMS – A PURPOSE OF TEACHING IN CONSTANTA MARITIME UNIVERSITY

In order to increase the knowledge of future marine engineers it is needed to bring closer the practical skills with theoretical gains. In Constanta Maritime University, Marine Refrigerating Plants is a discipline included in the curricula of future marine engineers. Practical learning can be also developed on low cost and effective means – as it will be proven in the paper. By using Coolselector 2, future marine engineers are trained to apply theoretical knowledge such as thermodynamics principles and to better understand the vapour compression cycle analysis, thermodynamic properties of refrigerants or the importance of the main components of a basic vapour compression refrigeration cycle. At the end of the practical session, students will gain important skills specific to theoretical aspects of refrigeration and thermodynamic processes and real situations.

Carbon dioxide evaporation heat transfer coefficient prediction in porous media using Machine learning algorithms based on experimental data

The prediction of the internal heat transfer coefficient during evaporation is vital for vapor-compression refrigeration and closed-loop power cycles. Accurate measurements and understanding of CO2 heat transfer in porous evaporators are essential for optimal system design across various operating conditions. This study utilizes a reference dataset derived from previous experiments that investigated the impact of porous evaporators on CO2′s internal heat transfer coefficient under sub-critical conditions, employing gravel sand as the porous medium. The dataset encompasses key factors: gravel sand porosities ranging from 39.8 % to 44.5 %, evaporator inlet pressures between 3700 and 4300 kPa, CO2 mass flow rates from 10.7x10-5–18x10-5kg.s−1, and porous tube effective diameters spanning 1.53 x10-3–3.4x10-3m. Employing three machine learning techniques (SVM, GPR, OBEM), the study predicts the internal heat transfer coefficient using regression models. The models’ predictions are analyzed and compared to expected values for validation, evaluating their performance using four statistical criteria. Results indicate SVM, GPR, and OBEM models achieved RMSEs of 1.5471, 1.8212, and 3.6978, respectively, while MAE errors were 1.1479, 1.2418, and 2.9787, respectively. Comparison with the dimensional analysis method reveals the effectiveness of the proposed models in accurately predicting internal heat transfer coefficients. The models exhibit low uncertainty and maintain prediction quality on an extended dataset without overfitting concerns. Overall, this research contributes valuable insights for designing heat exchangers and systems in vapor-compression refrigeration and closed-loop power cycles.