High Superheat Research Articles

Critical radius deviated from Leidenfrost state of droplets on liquid layer

The levitation of Leidenfrost droplets on liquid pool is fascinating, but its final stage is lack of understanding. Here, we found that a droplet levitated on liquid layer eventually deviated from Leidenfrost state once its radius was lower than a critical radius due to evaporation. The critical radius of ethanol droplet deviated from Leidenfrost state on silicone oils with a thickness ranging from 2.0 to 15.0 mm was determined by experiment. The influences of the initial radius of droplet, viscosity, and thickness of liquid layer on critical radius were analyzed. In addition, the critical radius decreases with increase in superheat for ΔT lower than 25.0 °C, whereas it does not significantly vary after ΔT exceeding 25.0 °C. The bottom temperature Tb of droplet does not approach to saturation temperature even under a high superheat. The experiment found that Marangoni convection existed inside droplet. Based on a theoretical model considering Marangoni convection, the reason for droplet deviated from Leidenfrost state was explained. These findings are helpful for understanding the final state of Leidenfrost droplet on liquid layer and would provide a potential practical application such as extinction of oil pool fires with liquids.

Performance investigation of a high-temperature absorption-compression heat transformer with liquid refrigerant injection

The high-temperature heat pump technology is a promising approach to utilizing low-temperature waste heat while meeting industrial demands for high-temperature heat. However, existing high-temperature heat pump systems face challenges including insufficient temperature lift, system complexity, and rapid performance degradation at high output temperature. This paper proposes a novel, structurally simple high-temperature absorption-compression heat transformer, aiming to recover heat below 100 °C and achieve a large temperature lift above 70 °C. The system is analyzed from the aspects of energy, exergy, key parameters that impact its performance, and techno-economic comparison. Results show that injecting liquid refrigerant into the compressor chamber mitigates high superheat effects on the compressor, allowing for higher absorption pressure. The system achieves an output temperature of around 170 °C, with the compressor outlet temperature below 120 °C. Moreover, some liquid refrigerant evaporates by absorbing compressor superheat. This reduces power consumption of the compressor, heat input of the evaporator, and overall system exergy loss. Thus, the proposed system demonstrates improvements of 6.5 % and 7.5 % in coefficient of performance and exergy coefficient of performance, respectively. Besides, it is feasible to achieve absorption temperatures exceeding 170 °C by increasing the solution concentration or the pressure ratio, with the former being the more cost-effective approach. Furthermore, the specific costs of the proposed system are more than 20 % lower than those of other types of high-temperature heat pumps, demonstrating its superior economic feasibility.

Experimental investigation on pool boiling heat transfer enhancement using reticular bi-conductive surfaces

Pool boiling has been a research focus over the decades because of its superior heat dissipation performance. This paper provides insights into the underlying pool boiling enhancement mechanism of bi-conductive surface which is an effective modification approach. In this study, a new type of bi-conductive surface is created by embedding a reticular structure made of low-conductive epoxy into a high-conductive copper substrate. Because of the enormous difference in conductivity, the heat flux almost exclusively flows through copper. The existence of epoxy creates periodic spatial surface temperature variations and induces ordered liquid and vapor paths. The influence of the width and depth of epoxy on pool boiling is studied by conducting heat transfer experiments. According to the results, all bi-conductive surfaces exhibit critical heat flux (CHF) increases in copper zone and the improvement is more significant with the increase of epoxy width. However, the depth of the epoxy has no significant effect on pool boiling. At low superheat, the heat transfer coefficient (HTC) in copper zone is relevant to the thermal flow rate of the independent copper zone. At high superheat, bi-conductive surfaces with wider epoxy have higher HTC in copper zone. The results show that for the bi-conductive surface with 0.75 mm width of epoxy, replacing 44% of the high-conductive copper with low-conductive epoxy results in a 270% CHF increase in copper zone and a 108% increase in the maximum total effective thermal power.

Influence of cavity geometry on the bubble dynamics of nucleate pool boiling

Nucleate pool boiling is known for its exceptional heat transfer coefficients, with the use of cavities further improving bubble nucleation and heat transfer rate. To promote this heat transfer enhancement technique, a thorough understanding of the influence of cavity geometry on single bubble dynamics is required. The influence of depth and radius of cylindrical and conical cavities on the bubble dynamics of nucleate pool boiling of R1234yf were numerically investigated. The cavity radius was varied between 50 and 400 μm and the cavity depth between 100 and 1000 μm at a fixed heat flux of 28 kW/m2. It was found that the maximum equivalent diameter prior to departure was constant for cavities with radii smaller than 120 μm, while it increased linearly when increasing the cavity radius further. Cylindrical cavities exhibited high stability regardless of cavity radius or depth whereas conical cavities showed a decrease in vapor retention with increasing cavity angle. During the necking phase, the bubble interface became pinned at the cavity edge, depending on conical cavity angle, implying that smaller radii allowed for enhanced surface rewetting. Conical cavities could be considered as cylindrical cavities when the cavity angle was less than a quarter of the interface contact angle. When translating the single cavity findings to cavity array design, cylindrical cavities were recommended as they allowed for stable bubble behavior. For increased nucleation zones and rewetting, a sub-critical radius was recommended. Wider cavities were recommended for high superheat conditions as larger bubbles could enhance bubble growth.

Numerical study on 3D nucleate boiling over an oscillating base plate with high wall superheat and multiple nucleation sites

This paper is on 3D numerical study on nucleate boiling at high wall superheat of 13.5 K, over an oscillating base plate, using a sharp interface dual-grid level set method (SI-DGLSM)-based in-house code. These studies are performed for two nucleation sites, with two configurations of the computational domain. A micro-layer evaporation model has been used to enable 3D simulations on a coarser grid and higher wall superheats. The size of the computational domain is chosen based on the nucleation site density, reported in experiments, and the present numerical results compare well with the published experimental results; for a stationary horizontal plate under atmospheric pressure. For nucleate boiling on an oscillating plate, bubbles departing from both the nucleation sites synchronize and lock-on with the plate oscillation. Enhancement in heat transfer coefficient is observed in presence of base plate oscillations, which is attributed to increase in rate of phase change as compared to increase in rate of convective heat transfer. Non-uniform distribution of nucleation sites on the base plate leads to formation of local convergence zones on the plate, which in turn affects the motion of bubbles as they rise within the computational domain. The present work is significant as it presents 3D simulations for nucleate boiling under the effect of base plate oscillations—as a promising technique for heat transfer enhancement.

Three-dimensional simulation and artificial neural network optimization of a flat plate heat pipe for the cooling of telecommunication equipment

To address the significantly increased heat dissipation requirements of 5G base stations, a flat plate heat pipe (FPHP) of 500 × 200 × 3 mm3 comprising of 19 independent channels was developed. To solve the problem of simulation distortion due to the high superheat of the initial boiling of the working fluid in the ultra-long channel of 490 × 5 × 1.5 mm3, this paper adopts the Lee model considering the superheat, and simulates based on the Volume of fluid method, and the simulation results are compared with the experiments, and the average relative error is less than 5 %. The FPHP was explored at 100–500 W heat power and filling ratio of 30 %-70 %, depending on the application scenarios of the base station. By partitioning the 19 channels and calculating the thermal resistance of the FPHP based, an Artificial neural network (ANN) model with 10 hidden layer nodes is established based on 675 data sets. The results showed that the mean square error and correlation coefficient were 1.30 × 10-5 and 0.9990. The ANN prediction results are validated, the mean relative error is 5.4 %, demonstrating that the ANN model can be an effective tool to optimize the FPHP for the telecommunication equipment.

Experimental study on generation and growth of bubbles in external flash-boiling spray regime

High-performance liquid propulsion systems, such as hypersonic scramjets and liquid rockets, utilize regenerative cooling by employing fuel as a coolant to shield components from thermal damage. The flash-boiling spray, resulting from superheated liquid injection and prevalent in regenerative cooling, has received considerable research interest for enhancing atomization and combustion efficiency. In this study, superheated water jets at various temperatures between 30 and 90 ℃ were injected into a vacuum chamber at different pressures above 5 kPa, and their spray patterns were acquired through diffusive backlight imaging. Images were analyzed to determine key parameters, such as bubble nucleation flux, bubble growth rate, and spray angle, in an external flash-boiling spray regime. The estimated nucleation flux agreed well with the predictions the model based on nucleation theory in the region of sufficiently high superheat. The dynamics of bubble growth was investigated and explained using a bubble growth model. The jet-expansion energy was computed by measuring the spray angle and compared with the available energy. By mapping the evolution of flash-boiling spray in relation to relevant nondimensional numbers, the study underscores the necessity of accounting for the pressure inside the injector to adequately model bubble generation inside the injector. Furthermore, this study substantiates the feasibility of visually recognizing and analyzing individual bubbles in the external flash-boiling regime, thereby enabling the generation of experimental data crucial for the validation of bubble nucleation and growth models.

Inhibiting the Leidenfrost Effect by Superhydrophilic Nickel Foams with Ultrafast Droplet Permeation.

Inhibiting the Leidenfrost effect has drawn extensive attention due to its detrimental impact on heat dissipation in high-temperature industrial applications. Although hierarchical structures have improved the Leidenfrost point to over 1000 °C, the current performance of single-scale structures remains inadequate. Herein, we present a facile high-temperature treatment method to fabricate superhydrophilic nickel foams that demonstrate ultrafast droplet permeation within tens of milliseconds, elevating the Leidenfrost point above 500 °C. Theoretical analysis based on the pressure balance suggests that these remarkable features arise from the superhydrophilic property, high porosity, and large pore diameter of nickel foams that promote capillary wicking and vapor evacuation. Compared to solid nickel surfaces with a Leidenfrost temperature of approximately 235 °C, nickel foams nucleate boiling at high superheat, triggering an order of magnitude higher heat flux. The effects of the pore diameter and surface temperature on droplet permeation behaviors and heat transfer characteristics are also elucidated. The results indicate that droplet permeation is dominated by inertial and capillary forces at low and high superheat, respectively, and moderate pore diameters are more conducive to facilitating droplet permeation. Furthermore, our heat transfer model reveals that pore diameter plays a negligible role in the heat flux at high surface temperatures due to the trade-off between effective thermal conductivity and specific surface area. This work provides a new strategy to address the Leidenfrost effect by metal foams, which may promise great potential in steel forging and nuclear reactor safety.

Solidification Microstructure and Segregation in the Medium-carbon Steel Cast with a Laboratory-scale Local-chilled Mold

In the industrial steel manufacturing process, such as continuous casting and ingot casting, macrosegregation occurs due to the effect of bridging and contraction flow during the middle and end periods of solidification. Since the macrosegregation results in a nonuniform structure and eventually cracks, there has been a demand for technological development that does not cause macrosegregation in the casting process. In this study, model experiments using the medium-carbon steel cast with a laboratory-scale local-chilled mold at different superheating have been carried out to investigate the relationship between solidified structure and macrosegregation occurred by local bridging during casting. The morphology of shrinkage porosities and dendrite structures was observed. The concentrations of the alloying elements were analyzed for macrosegregation by an electron probe micro-analyzer. Chill plates successfully formed the columnar dendrite bridging area and the columnar dendrite shell in the sample with high superheating. During solidification, the negative pressure of the region below the bridging increased, and the concentrated contraction flow flowed into the bottom of the large shrinkage porosity. Finally, V segregation was formed in the bridging area, large shrinkage porosities remained below the bridging area, and point-like or band-like positive macrosegregation occurred in the interdendritic region between columnar dendrites and equiaxed dendrites below the bridging. In comparison, a lower casting temperature increased the grain density and formed shrinkage porosities that were smaller in size but larger in number and more dispersed.

Rapid bubble growth within 200 μs in flat microchannels under “hot spot” heating conditions at ultra-high heat fluxes

The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter “hot spots” characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized “hot spot” heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work.

Electric field inspired alternating surface wettability for enhancing pool boiling heat transfer performance: A lattice Boltzmann method study

Electric field and surfactants have been proven to potentially alter surface wettability on demand. In this work, alternating wettability inspired by an electric field has been used to enhance boiling heat transfer performance in terms of critical heat flux (CHF) over a rough surface by using two-dimensional lattice Boltzmann simulations. In alternating wettability, the surface wettability changes with time between hydrophobic and hydrophilic. The effects of the parameters, including alternating wettability time-period and wettability contrast on the boiling process have been revealed. The simulation results show that for the alternating wettability: when surface is hydrophobic bubble expansion is faster, and then wettability transition to hydrophilic quickly retracts three-phase contact line leading to faster bubble shrinking as compared to constantly hydrophobic and constantly hydrophilic surfaces. Furthermore, the rapid shrinking of the bubble tends to disintegrate vapor film on the surface at high wall superheats, resulting in a 53% enhancement in CHF compared to a constantly hydrophilic surface. The bubble departure takes much longer time for too small/large time-periods of alternating wettability. The CHF increases with time-period, becomes maximum at a certain value and then reduces. The CHF also increases with wettability contrast, becomes largest for hydrophilic and hydrophobic contact angles 55o and 107o, respectively, and decreases with further increase in wettability contrast due to high potential for vapor film formation.

Experimental study on the flow characteristics during liquid nitrogen leakage

ABSTRACT The accidental leakage from tanks and pipelines is one of the main risks in cryogenic superheated liquid storage and transportation. To investigate the effects of superheat degree and jet pressure on the leakage characteristics, the corresponding liquid nitrogen (LN2) leakage experiments were carried out under different conditions. In this study, a rectangular nozzle ( 1 mm × 3 mm ) was applied to imitate the crack in typical release scenarios. The dynamic evolution process of jet pattern was also investigated. Three jet patterns including the mechanical breakup jet, the partially flashing jet, and the fully flashing jet were identified in the stable stage. The results indicate that the jet angle rises with the increase of superheat degree. At high superheat degree, the jet pressure presents a more significant influence on the jet angle. While the superheat degree decreases, its influence on the jet angle reduces. The leakage mass flow rate declines with the increase of superheat degree and rises with the increase of jet pressure. Furthermore, a new discharge coefficient correlation suitable for superheated LN2 leakage is developed, and its prediction error is within±3.8%. The new correlation is a good tool to predict the mass flow rate during LN2 leakage process.

Model of multiphase flow, superheat transport, and particle capture in a steel continuous caster

A model of turbulent multiphase flow, heat transfer, and particle entrapment during continuous casting of steel is presented. The model includes the top 7m of the vertical and curved strand, and considers the effects of argon gas injection and thermal buoyancy. RANS model flow results are compared with LES. Lagrangian particle transport through the Eulerian-Eulerian multiphase flow field from the k-ɛ RANS model is based on random walk and features advanced particle capture criteria, improved from previous work by including anisotropic turbulent velocity fluctuations near the walls. Particle capture via 3 mechanisms is included: capture by solidified hooks at the meniscus, entrapment between dendrites, and engulfment by surrounding large particles. The fluid flow and bubble/inclusion capture results are validated with plant measurements, including nail board dipping tests and ultrasonic tests of particle locations, and good agreement is seen. The superheat has negligible effect on flow in the mold region but causes complex flow in the lower strand by creating multiple recirculation zones due to the thermal buoyancy. With high (30 K) superheat, this leads to less penetration, and slightly fewer and shallower capture of particles. Lower (10 K) superheat may enable significant top surface freezing, leading to very large internal defect clusters. Lower superheat also leads to deeper meniscus hooks, and more surface capture. Capture bands occur near the transition from vertical to curved, where downward liquid flow velocity balances the particle terminal velocity.

Characterizing effect of particle size on flow boiling in sintered porous-microchannels

Flow boiling in porous microchannels draws extensive attention recently owing to its great potential in high heat flux applications. Subcooled flow boiling of deionized and degassed water was carried out to characterize transport performance. Porous microchannels were sintered from six-sized dendritic-like copper particles: 10, 30, 50, 90, 120 and 150 μm with the same layer thickness of 400 μm. The dimensions of 23 parallel porous microchannels are 600 μm × 1200 μm × 2800 μm (width × depth × length). The inlet subcooling degree and mass flux was maintained at 40 K and 142 kg/m2·s, respectively. Experimental results show that the particle size has great effects on the heat transfer coefficient (HTC) and Critical heat flux (CHF) of flow boiling in porous microchannels. Medium particle size (90 μm or 120 μm) remains higher performance in HTC and CHF. In heat fluxes between 60∼160 W/cm2, average HTCs of both PM-90 and PM-120 could reach almost 120 kW/(m2·K) or so. The visualization study reveals that the underlying heat transfer mechanism in porous microchannels is dominated by the nucleate boiling in low heat flux and then, by the thin film evaporation from moderate to high heat flux. Moreover, two boiling crisis phenomena have been observed: (a) in microchannels sintered from optimum size particles, the rewetting-dryout cycle has been well established and sustained without being interrupted by occasionally explosive boiling events, indicating capillary limits; (b) in non-optimum samples, high wall superheat in the rear part usually leads to explosive boiling persistently, which interrupts periodic oscillation modes with heat flux exceeding to moderate level and results in CHF conditions. An optimal ratio of bottom wall thickness and particle diameter, δ/d, is found in the range of 3∼5 for the flow boiling in sintered porous microchannels.

Dynamics of a Vapor Bubble in Film Boiling and the Superheat Effect

This study aims at developing an improved numerical simulation of the film boiling regime phenomenon to understand and visualize the growth of vapor bubble at a heated surface during low and high superheats. The simulation of the bubble dynamics including the bubble growth, departure, coalescence, rising, and frequency of detachment under different wall superheats is numerically investigated. The continuity, momentum, and energy equations are solved for the two immiscible fluids phases using the finite volume method. The phase change model and the results exhibited a good agreement with the theoretical models. The obtained results show that the velocity of bubble growth and its frequency of emission promotes heat exchange. It is found that the shape of a bubble has been influenced by the wall superheat. It is also found that the high superheat generates a large amount of steam in which the steam bubble takes the shape of a fungus. So, a clear correlation exists between heat transfer and the frequency of detachment. As long as the frequency is greater, the heat transfer increases. Most of the heat transfer is induced by the liquid movements associated with the vapor bubble detachment.

Eggshell waste separation process assisted with pressure-vacuum: Process conditions and optimization.

Eggshells and eggshell membranes have high-value recycling applications and have been widely used in pharmaceutical, chemical, and food research. The separation of eggshells and eggshell membranes is a prerequisite to efficiently using both. Therefore, the pressure-vacuum experiment equipment was designed. In this study, research on the separation of eggshells and eggshell membranes from waste eggshells using the pressure-vacuum experiment equipment was carried out. The flash evaporation experiment process controlled the experimental factors to obtain a sufficient moisture content between the eggshell and eggshell membrane with vigorous flash evaporation. The effects of experimental factors such as superheat (5-10°C), temperature (50-70°C), initial pressure (0.6-0.8MPa), pressurization time (0-40min), and particle size (6-8mm) on the separation rate were investigated in the pressure-vacuum experiment process. Through single-factor and orthogonal experiments, it was found that the separation rate was most affected by changes in temperature, initial pressure, and particle size, followed by the interaction of temperature and particle size. The experimental results suggested that the optimum separation of eggshell membranes from eggshells was achieved at higher superheat, higher temperature, higher initial pressure, medium pressurization time, and smaller particle size. Through optimization by response surface methodology, the optimal conditions for the separation of eggshells and eggshell membranes using the flash evaporation method were determined as 15°C of superheat, 70°C of temperature, 0.8MPa of initial pressure, and 6mm of particle size. Flash evaporation method is an effective and environmentally friendly method, which provides a new solution for the recycling of waste eggshells. PRACTICAL APPLICATION: In this study, pressure-vacuum experiment equipment was utilized to reuse of waste eggshells, and an innovative and environmentally friendly method of eggshell membrane and eggshell separation was established. The pressure-vacuum experiment equipment has a simple structure and low energy consumption. The results of flash evaporation experiments are instructive for further in-depth studies on the separation of eggshells and eggshell membranes. Furthermore, the separation of eggshells and egg membranes by flash evaporation is of great research value. Most importantly, the separated eggshells and eggshell membrane are available for high-value applications in food, chemical, and biological fields.

Break the superheat temperature limitation of induction skull melting technology

Overcoming the limitation of low superheat temperature problem is key for the applications of the induction skull melting technology in many important high melt point metals used in fields such as aerospace. The present paper presents an advanced numerical study to explain the mechanism of melt superheat temperature. A coupling model between electromagnetic, thermal, phase and fluid flow fields was established, which accurately predict the temperature and meniscus shape in the research domain. The simulation results show that the forced convective heat transfer caused by electromagnetic force in the melt and the current passing through phenomenon between molten melt and crucible were the key to influence the superheat temperature and electrical efficiency. Furthermore, it was discovered that convection suppression in the melt by employing an external DC coil and increasing the thermal resistance between the skull and the crucible could increase the melt superheat temperature effectively. Increasing the thermal resistance between the skull and the crucible also leads to an increase in the superheat temperature, which could be achieved by enhancing the interface roughness. The present work would shed lights on the design of new generation induction crucible to obtain high superheat temperature melt.

Collaborative mechanism of the plume expansion and fluctuations in the near-field under non-flash and flash conditions

The objective of this study is to unravel the driving mechanisms behind the plume expansion and fluctuations in the near-field. A collaborative experimental and numerical study was performed under various operating conditions. Spray width in the near-field was quantified using diffused back illumination imaging, and microscopic characteristics were captured by phase Doppler anemometry; the modified models were then used to obtain internal flow patterns. The dominant factors that account for the lateral expanding of the plume are examined, and the thermal factors contribute to the plume expansion under high superheat degree conditions. A new parameter is proposed to describe the lateral spreading variations on various operating conditions for different types of fuel. Aerodynamic and internal flow begins to dominate the spray characteristics when superheat degree is not strong enough. The vortex transport is investigated, functioning as a bridge connecting with the cavitation evolution, gas ingestion, and turbulence characteristics. The connection between distinct spray characteristics and operating parameters, fuel properties, and various internal flow patterns is established, supplementing our knowledge about the optimization of injector geometry and injection strategy.

Boiling heat transfer of nanofluids on the graphene membrane: A molecular dynamics simulation

The molecular dynamics method was used to study the boiling heat transfer of the base fluid (consisting only of water molecules) and nanofluid film on the graphene membrane. The nanofluid consists of water molecules and Cu nanoparticles of three sizes (0.8 nm, 1.2 nm, and 2 nm). Compared with the boiling heat transfer of the base fluid on the graphene membrane at high superheat, the influence of nanofluids with different particle sizes on boiling heat transfer was studied. The effects of the particle size, the water temperature at the radial distance of the particle surface, the molecular mass density distribution, the evaporation rate, and the heat flux on the boiling are analyzed. The heat transfer at the interface between nanoparticles and graphene mainly relies on phonons. The results show that the nanoparticles act as an indirect heat source to heat the surrounding water molecules and strengthen the boiling heat transfer at the solid-liquid interface, and the thermal conductivity at the interface increases with the increase of particle size. The atomic vibration frequencies between the graphene membrane and nanoparticles are well coupled in the low-frequency region, which enhances the boiling heat transfer.

Influence of operating parameters on the COP of an R600 high-temperature heat pump

Process heat supply by high-temperature heat pumps exploiting waste heat is an effective method for enabling the decarbonisation of industrial processes. Key aspects for the application of this technology are the supply temperature levels and efficiencies that can be achieved. This work investigates an R600 (n-butane) high-temperature vapour compression heat pump (HTHP) capable of utilizing waste heat at temperature levels between 40 and 60°C and lifting it to supply temperatures between 110°C in sub-critical and 160°C in trans-critical operation. The HTHP prototype is designed as a single-stage cycle, comprising an inverter-driven suction-gas-cooled reciprocating compressor, low-pressure accumulator and internal heat exchanger (IHX) for suction gas superheating. The influence of the operating parameters high pressure and suction gas superheat (“internal parameters”) plus the heat source temperature, the heat sink inlet and outlet temperatures and the compressor speed (“external parameters”) on the COP and heating capacity are studied using data from both the prototype and the simulation model. The optimum high pressure depends on the operating conditions, an increase of suction gas superheat increases the COP. In sub-critical operation, a heating capacity of 30.7 kW at a COP of 4.4 is reached at a source inlet temperature of 60°C heating the sink from 80 to 110°C. Trans-critical operation enables a heat sink outlet temperature of 160°C, offering a heating capacity of 24.2 kW at a COP of 3.1.