Heat Rejection Research Articles

High-pressure liquid refrigerant injection for reciprocating compressors

Isentropic (adiabatic) compression of superheated vapor in vapor compression refrigeration cycles usually results in some discharge superheat, i.e. the discharge temperature exceeding the condensing temperature. Large discharge superheat reduces both operating range and compression efficiency. Sub-isentropic compression with heat rejection to the heat sink is therefore superior to adiabatic compression, as it results in lower discharge superheat and improved efficiency. However, significant temperature reduction by heat transfer through the working chamber walls is difficult to facilitate in common refrigeration compressors, particularly reciprocating compressors. Refrigerant and oil injection is a well-established method for reducing temperatures and increasing efficiency in screw and scroll compressors, as well as multi-stage systems. In comparison, effective methods for temperature reduction in single-stage reciprocating compressors are fairly limited. Therefore, a concept for high-pressure liquid injection is proposed. By means of a pump and injection valve, high-pressure liquid refrigerant is injected into the working chamber and atomized. The compressed gas is cooled through the evaporation of the injected liquid. The injection valve allows for control over the injection timing and quantity, enabling adjustment of the temperature profile, which is a major distinction from existing refrigerant injection methods. The coupled compression-injection process is evaluated for various refrigerants using an energetic chamber model. The injection is found to be particularly suitable for refrigerants with a large evaporation heat and a shallow dew line slope. For the example of ammonia, a theoretical reduction in compression work of 8.9% at an evaporation temperature of −10 °C and a condensing temperature of 45 °C is found.

Lightweight Heat Pipe Radiator for Nuclear Reactor Power Systems on Lunar Surface

Heat rejection radiators of nuclear reactor power systems for space exploration and for planetary surface power are the largest component by volume and mass, depending on the radiator’s design and surface average temperature. This work developed designs for lightweight radiator modules for waste heat rejection on the lunar surface at a surface average temperature of 600 K. The modules each have a cesium (Cs)–titanium (Ti) heat pipe (HP) and highly oriented pyrolytic graphite (HOPG)/Ti heat spreading fins. The assembled panels of 10 Cs-HP modules hydraulically coupled in parallel are armored with carbon-carbon composite to protect against impacts by micrometeoroids and space debris for 10 years. The performance of the developed armored radiator panels is much superior to the current state of the art, with an areal density of 2.98 to 3.6 kg/m2, specific power of 3.36 to 3.98 kW/kg, rejected thermal power of 56.3 to 96.3 kW, and rejected power density of 7.56 kW/m2.

Experimental investigation of multi-ejector CO2 heat pump system with and without IHX

Environmental degradation due to the use of synthetic refrigerants should motivate end users to request natural refrigerants to secure their businesses from unforeseen negative ecological impacts. Further, using natural refrigerants as working fluids for HVAC requirements helps achieve zero CO2 emissions from this sector. The CO2 heat pumps are energy efficient units providing simultaneous heating and cooling. Implementing performance enhancing components like ejectors and/or internal heat exchangers (IHX) is sometimes beneficial. This study experimentally evaluates the ejector performance in a multi-ejector CO2 system of 33 kW cooling capacity, with and without an IHX, at different gas cooler exit temperatures. Evaporator and gas cooler exit temperatures are varied from 4 to 10 °C and 34 to 46 °C, respectively. These temperature ranges suit the simultaneous cooling (AC) and heating demands in various buildings and food- and beverage industries. At the gas cooler exit temperature of 42 °C, the system COP is improved by 12 % with an IHX, whereas the ejector efficiency increased from 14 to 18 % without the IHX. The ejector efficiency and entrainment ratio also increased at elevated heat rejection temperatures. However, as expected, the system COP decreased. Therefore, the heating system must be adapted to avoid high gas cooler exit temperatures.

An experimental study exploring heat transfer enhancement in tube in tube heat exchanger with pulsating flow

Rationalizing power consumption is crucial in air conditioning and refrigeration design. This study focuses on improving the coefficient of performance (COP) by applying pulsating flow. Experimental trials compared continuous counter and counter pulsating flow to continuous parallel flow, showing a 10% reduction in fluid temperature with continuous counter flow and a 20% reduction with counter pulsating flow. Sensible water heat rejection increased by 10.03% and 19.78% with continuous counter flow and counter pulsating flow, respectively. Overall heat transfer coefficient improved by 57.5% and 75% with continuous counter flow and counter pulsating flow. Effectiveness increased by 4.55% and 10.6%, while COP increased by 4.52% and 13.4% with continuous counter flow and counter pulsating flow, respectively. This research demonstrates the potential of pulsating flow techniques to enhance air conditioning and refrigeration efficiency.

Performance analysis and optimization of thermal barrier coated piston diesel engine fuelled with biodiesel using RSM

The current research investigates diesel and simarouba biodiesel blends (10%, 20%, & 30% by volume) in conventional and Low Heat Rejection (LHR) diesel engines, each rated at 4.4 kW. While optimization techniques like Response Surface Method and Taguchi have been extensively studied, the impact of LHR and optimization on LHR engine performance and emissions is rarely explored. Converting the conventional engine to LHR involved applying 300 μm of stabilized zirconia to the piston crown to enhance combustion efficiency. Performance and emissions were analyzed at rated injection pressure (200 bar) and timing (23° before top dead center - btdc). Experiments were continued on LHR engine by varying injection timings (advancing - 26°btdc and retarding - 20°btdc). Advanced injection timing showed significant improvement in performance of Low heat rejection engine. MINITAB statistical tool is used to optimize engine performance using Response Surface Method. The 20% blend showed improved performance in both engines. The optimum values for Low heat rejection engine responses are 26.8%, 0.32 kg/kW-h, 0.018%, 59.59 ppm, and 1419.03 ppm for brake thermal efficiency, brake specific fuel consumption, carbon monoxide, and unburnt hydrocarbons, respectively. Confirmation experiments aligned well with model predictions, indicating the potential of LHR engines to enhance thermal efficiency and reduce emissions.

An Ultralight Capillary-Driven Heat Pipe.

Capillary-driven heat pipes are an effective thermal solution for compacting electronic cooling systems. We advance such a heat pipe thermal solution with ultralightweighting for mobile applications. In our advancement, the envelope that encapsulates the phase-change process of a working fluid is fabricated via electroless plating being ∼40 μm thick. Furthermore, the wick structure that transports condensate to a heat source via capillarity is also electroless-plated onto the envelope's inner surfaces, creating a 100-μm-thick, microporous layer. This wick structure is sequentially superhydrophilized by blackening that forms a nanotexture on the microporous wick layer. An effective density of our prototype ultralight heat pipes (uHPs), as a measure of lightweighting, indicates, on average, a remarkable 73% weight reduction of commercial counterparts with sintered copper powder wick in similar exterior dimensions (e.g., ∼2.7 g, compared to ∼10.0 g) while providing equivalent heat spreading. Furthermore, the uHP operates at a 25% lower evaporator temperature, due to additional heat rejection to the surroundings through the ultrathin-walled envelope and wick.

Experimental comparison of finned tube heat exchangers for heat rejection under natural convection conditions

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions.

Experimental analysis into the effect of heat exchanger sizing on charge optimization in an R410A chiller with a dedicated subcooler

There is an increased focus on the amount of refrigerant used in the HVAC&R systems due to the ongoing phase-down of high GWP refrigerants. In the current work, charge optimization is conducted with a variable speed compressor in an R410A chiller having a nominal cooling capacity of 8 kW, which is installed with a condenser, receiver and dedicated subcooler. This study also provides for the first time insights into how a receiver and a dedicated subcooler can be used to not only obtain optimum subcooling but also account for any potential refrigerant leaks by holding additional charge in the receiver. Results indicate that an ideal charge optimization curve can be achieved by sizing the condenser and subcooler to ensure only the optimum subcooling happens in the subcooler. The impact of dedicated subcooler and condenser sizing on charge optimization as well as the system, and seasonal performance is also studied. This analysis helps in understanding how these components should be sized and when the diminishing returns occur from oversizing. This work also shows that when the system is operating with enough refrigerant to fill the receiver with saturated liquid refrigerant, using a dedicated subcooler decouples the cooling capacity increment from power consumption increase typically observed with increasing subcooling. Results show that for the current system investigated increasing the dedicated subcooler area beyond 8 % of the total available heat rejection area will not yield any additional seasonal performance improvement.

Performance evaluation of low heat rejection diesel engine operated with biofuels under-selective catalytic reduction

Vehicle emissions are responsible for about 30% of all air pollution in the world. Vehicle emissions can be significantly reduced through the use of selective catalyst reduction (SCR). The present work emphasizes the impact of thermal barrier-coated pistons on diesel engine performance as well as emission qualities. A Ni–Cr–Al–Y bond coat was applied to the tested engine piston that was 50 microns thick and a top coat that was 250 microns thick. These coatings were applied using the plasma spray technique to a combination of 2 mol. % of Gadolinium oxide (Gd2O3), 2 mol. % of Neodymium oxide (Nd2O3), 3 mol. % of Yttria (Y2O3), and continuing 93 mol. % of Zirconia (ZrO2). In a 4-stroke, 1-cylinder diesel engine, the testing was carried out utilizing diesel, Mahua, and Jatropha fuels with and without coating. The selective catalytic reduction technique was employed in the current test to reduce NOx emissions. The findings of this analysis indicate that the brake thermal efficiency of an insulated piston engine improved by 3.9%, and when JB 100 was chosen as the fuel, the insulated piston reduced brake-specific fuel consumption by 3.5% in comparison to the normal piston. In engines coated with SCR, hydrocarbon emissions were lowered by 20.1%, while carbon monoxide emissions were dropped by 13.4%. In comparison to the baseline engine, the oxide of nitrogen emissions were reduced by 39.1%.

Study on exhaust gas recirculation diesel engine using karanja oil methyl ester with low heat rejection in direct injection

Energy is a fundamental necessity for man›s life in the digital world today. The rapid depletion of fossil fuel resources forces rigorous alternative fuel analysis. Petroleum diesel can better replace vegetable oils, edible or motored today. The rapid depletion of fossil fuel resources forces rigorous alternative fuel analysis. Petroleum diesel can better replace vegetable oils, edible or not. Karanja may be a possible supplier of diesel fuel for non-edible oil substitution. Current combustion surfaces for pistons, valves, and cylinders have been filled with ceramic materials, which make the engine totally adiabatic (LHR). The performance of a biodiesel-powered compressing ignition (CI) engine may be further boosted by utilising the engine›s heat effectively and increasing thermal efficiency. Exhaust Gas Recirculation (EGR) is actually one of the most important methods of limiting NOx emissions in internal combustion engines. Explore the output with and without exhaust gas recirculation on a retarded timing engine with diesel and karanja oil methyl ester (KOME). The LHR with a retarded timing engine yielded improved thermal brake efficiency (TBE), decreased HC, smoke, and CO emissions, while increasing KOME›s NOx in comparison with an uncoated engine. As the EGR rate grew, NOx and BTE were reduced marginally with increased HC, CO, and smoke. 24.1 g/kw-hour CO, 10.1 g/kw-hour NOx, and 0.55 g/kW-hour HC were registered at 20 percent of EGR.

Thermal performance of a stack cooling radiator coupled with spray cooling for the future fuel cell electric vehicles

Proton-exchange membrane fuel-cell electric vehicles require an appropriate stack cooling radiator (SCR) system to achieve optimal performance and safe operation. However, SCR cannot achieve high heat rejection under high power demands and hot ambient conditions. Hence, either the SCR needs to be enlarged to manage heat rejection or its ability to reject heat should be enhanced. In this study, a novel low pressure drizzling-aided spray cooling system (SCR-DS) is proposed for the first time. The thermal performance results were compared with those of a high-pressure nozzle-aided spray system (SCR-NS) and a conventional SCR system (SCR-CS). Based on the results, the coolant entry temperature to the stack reduced by up to 1.78 °C, and 2.75 °C when SCR-NS and SCR-DS configurations were active, respectively. Among the various operating factors, the spray cooling performance was significantly influenced by air velocity, spray temperature, and spray flow rate. Furthermore, the experienced mean increments in the heat rejection rate were up to 71.7 % and 123.5 % for the SCD-NS and SCR-DS, respectively, compared to SCR-CS. More importantly, these configurations aid in reducing the size of the SCR by 36.2 % and 45.9 %, respectively, under the same overall heat transfer coefficient conditions. Overall, the SCR-DS configuration was more beneficial than the SCR-NS configuration in terms of thermal performance owing to the slow and steady spray on the SCR surface along with more spray coverage, which aided in better liquid convection and film evaporation.

Numerical investigation on cooling performance and control effect of a novel evaporation-driven cooling unit for Mars extravehicular space suit application

The special environment of Mars brings new challenges to the thermal control of spacecraft on Mars missions. The outstanding advantages of water evaporation cooling technology make it promising for use in the thermal control system of the Mars spacecraft. This paper proposes a novel concept of evaporation cooling hardware that integrates heat collection, heat transfer and heat rejection, namely evaporation-driven cooling unit. The evaporation-driven cooling unit has a simple structure, high evaporation efficiency and the potential to simplify the system topology. Furthermore, an expert hierarchical coordination control strategy applied to the Mars extravehicular space suit cooling loop with an evaporation-driven cooling unit as its heat sink is proposed. The dynamics model of this cooling loop is also constructed using the lumped parameter method. The numerical results have shown that the expert hierarchical coordination control strategy can achieve precise temperature control and reduce the water consumption of the system. The average water consumption rate during extravehicular activity is 0.6975 kg/h when the mean heat load is 572 W (including member metabolic heat and electronic waste heat). Compared to the open-loop case with the same input heat load, a typical extravehicular activity lasting 8 h will save about 11.5 g water if the expert hierarchical coordination control is applied. The proposed evaporation-driven cooling unit will provide more choices for heat rejection of extravehicular space suits. It is expected that the evaporation-driven cooling unit and expert hierarchical coordination control have potential applications in thermal control systems for other space vehicles in near-Earth orbit, on the surface of the Moon and Mars.

Behavior analysis of Low-Heat Rejection engine powered by biodiesel under varied exhaust gas recirculation ratios

ABSTRACT The key focus areas of the internal combustion engine architecture are energy effectiveness and greater thermal efficiency since 1/3 of the produced heat energy vanishes on the way to the atmosphere and the coolant. Experiments have looked into the viability of using tamanu biodiesel in a thermally insulated diesel engine. The engine was first put through its paces on regular diesel fuel as a comparison baseline. After that, the piston, exhaust, intake valve and cylinder head surfaces were coated with partially stabilized zirconia (PSZ) to act as thermal insulation for the engine. The engine coating was developed to improve the efficiency of the engine’s use of biodiesel blends by decreasing heat loss through the chamber walls with a thickness of 200 micrometers (µm). In addition, mixes of Tamanu biodiesel with diesel fuel were tested, including B10, B20, B30, and B100, to examine how these mixtures would influence the performance of the ceramic-coated low heat rejection (LHR) engine compared to the performance of the base diesel engine. Additionally, the impacts of different exhaust gas recirculation (EGR) ratios on the operation of biodiesel-powered LHR engines were analyzed. The zirconia insulated LHR engine running on B20 performed the best, with a thermal efficiency of 28.57% and a brake specific energy consumption that was considerably lower than that of the baseline diesel engine. In addition, the LHR engine for B20 reduces carbon monoxide (CO) and smoke emissions by 17% and 21.4%, respectively, compared to the B20-powered base diesel engine, although nitrogen oxide (NOx) emissions slightly increase. However, the addition of 15% EGR to the B20-LHR engine produced a minor increment in hydrocarbon (HC), CO and smoke emissions, corresponding to 19.7% and 25.4%, respectively. By implementing 15% EGR with B20-LHR, the NOx level is reduced by 25.4% due to diluting the air-fuel mixture. The combination of 15% EGR with B20 for a thermal barrier-coated engine offers significant improvement by increasing engine performance and reducing emission characteristics.

Dynamic simulation of a space lithium-cooled reactor system coupled with a He–Xe closed Brayton cycle

The lithium-cooled reactor with Brayton conversion power system mainly consists of fast reactor, intermediate heat exchanger, closed Brayton cycle (CBC) loops filled with Helium–Xenon (He–Xe) binary gas mixtures and heat rejection loop which radiate residual heat through heat pipes to outer space. In this study, a dynamic model of 100-kWe lithium-cooled reactor with Brayton conversion power system was established based on RESYS code, which include reactor core thermal-hydraulic model, six-group neutron kinetics model and decay heat model, heat exchanger model, Closed Brayton cycle model including turbine, compressor and rotating shaft, heat pipe radiator model etc. The nominal steady-state simulation is carried out to verify the correctness and accuracy of individual component model the integrated system model. Four typical transient conditions including accidental reactivity insertion, loss of coolant flow of primary loop, partial and total external load loss and a step reduction in the mass flow rate of heat rejection loop are simulated to analysis the transient characteristics and prove the safety features for those simulated scenarios. The results are helpful for the design, evaluation and operation of space liquid-lithium-cooled reactor power system with He–Xe closed Brayton cycle.

Energy, exergy and economic (3E) analysis of solar thermal energy assisted cascade Rankine cycle for reverse osmosis

A novel cascade Rankine cycle is proposed for treating brackish groundwater using a reverse osmosis system. The cascade RO system is arranged in a loop with a steam Rankine cycle (SRC) at the top and an organic Rankine cycle (ORC) at the bottom to provide high recovery, electricity-free, and scalable options. The system comprises a solar Scheffler dish for heat input, steam turbine and expander for work output, evaporative condenser and ORC condenser for heat rejection, SRC pump, ORC pump and RO high-pressure pump for work input devices, and RO module for water desalination. This study evaluated the thermodynamic design of the system along with energy, exergy, and economic analyses performed by considering eight working fluids such as R245fa, HFO1336mzz(Z), R1225ye(Z), R1224yd(Z), R1233zd(E), R1243zf, R1234yf, and R1234ze(E). The highest overall efficiency was found 14.08% with R1234yf, and the highest RO permeate flow rate was found with R245fa, HFO1336mzz(Z), and R1233zd(E), which require the lowest mass flow rate of ORC working fluid. The highest exergy destruction was found in the solar collector and RO unit. The treated water cost is estimated to be 0.89–0.924 $/m3 of permeate water.

Thermodynamic Modeling of a Supercritical Steam Rankine Cycle for an Integrated CSP-Nuclear System

Renewable technologies using solar input have varied electrical power production during periods of low solar irradiance caused by cloud coverage, seasonal changes, and time of day. Nuclear power plants can load follow, but due to low operating costs and high fixed costs, this capability is often not financially appealing. Implementing thermal energy storage (TES) within a synergistic solar and nuclear power cycle allows for storage during low demand periods and increased power production during high demand periods, effectively increasing dispatchability. In this paper, we examine the thermodynamic performance of an integrated system that includes concentrating solar power (CSP) and a lead-cooled fast reactor (LFR). These technologies are selected due to their similar operating temperatures, allowing for utilization of established TES technologies. Both the CSP and LFR system produce thermal power that is sent to a supercritical steam-Rankine cycle (SSRC). The SSRC model is designed to be implemented into a larger integrated energy system (IES) which contains multiple communicating models. The IES generates CSP and LFR heat profiles then utilizes the SSRC to output calculated metrics including power generation, heat rejection, and cycle efficiency.

Desiccant-assisted multistage air compression systems: Performance improvement and humidity control

Novel desiccant-assisted systems have been evaluated for reducing the work required in multistage air compression processes. While traditional inter-cooling is generally achieved by placing heat exchangers between compression stages, the systems herein proposed are based on harvesting heat rejection to drive a desiccant cooling system. This cooling effect is then used to cool the air between the compression stages below ambient temperature, and also to pre-cool the air prior admission to the first stage, allowing for a reduction of the compression work. In addition, desiccant dehumidification is also used to control the humidity of the air in the compression system. A model based on steady-state heat and mass transfer balances across all components is presented and computationally implemented for a two-stage compression system. Numerical results are analyzed in terms of a performance factor, showing that some configurations can achieve a reduction in compression work almost 60% greater than that achieved with conventional inter-cooling systems. A final test-case was also presented, based on inlet conditions extracted from weather data of Rio de Janeiro, Brazil, demonstrating that the proposed compression schemes could be used as a viable option for reducing compression work in multi-stage systems. The use of a waste-heat-driven desiccant-assisted system for reducing compression work and controlling humidity, is an important addition to applications of desiccant cooling.

Uncertainty Propagation in a Simplified Integrated Thermal Management System

An integrated generic plate model that describes single-phase transport combining heat acquisition and rejection regimes is investigated. Transient processes and input uncertainty propagation impacting temperature variations in this simplified, dimensionless integrated thermal management system are discussed. The heat acquisition plate is surface-heated on one side, with the heat being conducted into a liquid. The heated liquid is assumed to be adiabatically advected to the heat rejection plate. Convection boundary conditions are considered for both plates. The range of mean values of dimensionless input parameters, such as the Biot and Prandtl numbers, is determined by the choice of a particular plate metal and flowing liquid, and their stochastic nature is described by the Gaussian distributions. Adiabatic solutions for the dimensionless temperature variations in the plates are obtained for different temperatures and Reynolds numbers. As is shown, even an assumed small standard deviation for the input parameters may result in a drastic increase in the stochastic spread in the temperature variation in the heat rejection plate. Unlike the temperature variation across the acquisition plate, the temperature variation profile for the heat rejection plate is sensitive to the temperature of the incoming liquid and heat input at the acquisition plate.

Influences of Heat Rejection from Split A/C Conditioners on Mixed-Mode Buildings: Energy Use and Indoor Air Pollution Exposure Analysis

The heat rejected by outdoor units of split A/C conditioners can impact the ambient outdoor environment of mixed-mode buildings. Nevertheless, how this environmental impact may affect the space-conditioning energy use and indoor air pollution is poorly understood. By coupling EnergyPlus and Fluent, this study examines the effects of outdoor units’ heat rejection on the building surroundings, building cooling load, and indoor PM2.5 exposure of a six-storey mixed-mode building. The building had an open-plan room on each floor, with the outdoor unit positioned below the window. The coupled model was run for a selected day when the building was cooled by air conditioning and natural ventilation. Five mixed-mode cooling strategies were simulated, reflecting different window-opening schedules, airflow rates of outdoor units, and cooling set-points. The results indicate that compared with the always-air-conditioned mode, the mixed-mode operation could significantly mitigate the negative impact of heat rejection on space-cooling energy consumption. Increasing the airflow rate of outdoor units led to a lower increase in demand for space cooling and lower indoor PM2.5 exposure. If one of the six rooms needs to be cooled to a lower temperature than the others; choosing the bottom-floor room helped achieve more energy savings and better indoor air quality.

Modelling and optimization of a forced convection heat exchanger for Mars waste heat rejection for power cycle and cryocooling applications

A forced convection heat exchanger for waste heat rejection to the Martian atmosphere is modelled for both a power generation and cryogenic refrigeration application. The modeling is based on existing as well as recent, experimentally validated heat transfer and pressure drop correlations for low-Reynolds-number, moderate-Knudsen number finned tube arrays and represents the first detailed examination of optimal heat exchanger design and performance in this regime. These correlations were developed from heat exchanger performance data taken in Mars-like conditions and are described in a separate, companion work [1]. A heat exchanger model based on the effectiveness-NTU method is developed using these correlations and the heat exchanger geometry is optimized using an open-source solver to minimize the overall system mass for a range of candidate heat exchanger materials and heat transfer rates. The optimal heat exchangers generally consisted of shallow, staggered banks of 1–2 mm diameter tubes and 2–4 mm fin spacing, although in some conditions bare tube arrays were optimal. These geometries tended to be preferred due to their low relative pressure drop, a necessity given the power requirement of producing high pressure head in the thin atmosphere. Curve fits that capture the heat exchanger performance are generated based on the input conditions in order to develop a reduced order model that can be used to integrate the heat exchanger model with an existing recuperated Brayton cycle high-temperature gas cooled reactor (HTGR) power cycle model. This power cycle model is used to determine the impact of using convective heat rejection on optimal system mass and compare this approach to radiative heat rejection. For a 40 kWe power output, the optimal overall cycle mass is found to decrease by 78 % when using convective heat rejection compared to a radiator; this is due to the decreased mass of the heat rejection system as well as the lower optimal rejection temperature which results in a lower recuperator mass. The heat exchanger itself has a mass of only 37 kg, a frontal area of 12.7 m2, and requires 2134 W of fan power to operate. The system is shown to perform well over a wide range of Mars ambient conditions. A high-power liquid Oxygen (LOx) reverse Brayton cryocooler cycle is also modeled to determine the benefit of using convective heat rejection for in-situ resource utilization (ISRU) applications. Across a range of power source options and ambient temperatures, using a convective heat rejection system reduced total system mass by up to 43 % and frontal area by up to 95 % compared to a radiator and also reduced the mass-optimal cycle power draw by increasing the optimal cycle efficiency. However, the magnitude of the mass reduction depends strongly on the specific power of the electrical source; the forced convection system provides only a 7 % improvement over a radiator when coupled with a photovoltaic power source due to the increased power demand of the fan. Overall, this technology offers significant performance advantages over radiative heat rejection systems for waste heat rejection applications on Mars and therefore may significantly benefit future crewed Mars missions.