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Three-dimensional conjugate heat transfer models are built to predict the steady-state performance of microscale pin-fin and cross-flow heat exchangers with hydraulic diameters on the order of 100 μm. Modeling, meshing, and segmentation techniques are presented to allow for macroscale simulation of the microstructured devices. The effect of variation in geometric and flow parameters is investigated. Hydraulic and thermal predictions are compared to published experimental and extended beyond the limited range of test data to provide performance within a wide parametric range. A discussion of the dominating and relevant thermal transport mechanisms in both fluids and solid clarifies the routes to optimizing heat transfer in these small scale heat exchangers.

The power and thermal management system (PTMS) developed by Honeywell for aircraft is an integral approach combining the functions of the auxiliary power unit (APU), emergency power unit (EPU), environmental control system (ECS), and thermal management system (TMS). The next generation PTMS discussed in this paper incorporates the new more electric architecture (MEA) and energy efficient aircraft (EEA) initiatives. Advanced system architectures with increased functionality and further integration capabilities with other systems are included. Special emphasis is given to improvements resulting from interactions with the main engine, main electric power generation, and flight actuation. The major drivers for advancement are highlighted, as well as the potential use of new technologies for turbomachinery, heat exchangers, power electronics, and electric machines. More advanced control and protection algorithms are considered.

Electrohydrodynamic (EHD) forces acting on a fluid can be used to significantly increase the heat transfer coefficient with little power consumption or flow restriction. Experiments have shown promising results with simple configurations, but practical heat exchanger development is needed. Analytical development will establish design equations and aid in experimental data correlation. Small scale test units will provide experience in determining heat exchanger configurations and electrode options. Controller and high voltage power supply units will also need to be developed. Full-scale EHD heat exchanger systems can then be developed to demonstrate technology availability for military and commercial products.

Thermal management requirements for aerospace applications continue to grow while weight and volume allotments remain constant or shrink. Compact, high performance and lightweight heat transfer equipment is needed to meet these high heat flux removal requirements. Several innovative heat transfer enhancement techniques are being considered for development of thermal management components that will meet these challenging demands. Honeywell, under an AFRL funded program, is developing two new heat exchanger technologies; microchannel and advanced heat transfer surfaces to improve thermal management systems for a fuel-to-air heat exchanger. Heat transfer systems in military aircraft are increasingly using fuel as a heat sink. Heat transport loops containing several fuel-to-liquid heat exchangers are used to cool electronics, engine oil, hydraulic oil, and elements of the thermal management system.

Advances in aircraft technology have brought about a necessity for new aircraft thermal management architectures in order to maintain reasonable cost, size, weight, and power requirements of the overall thermal system. Two-phase cooling technologies such as vapor-compression systems have demonstrated significant benefits and offer a serious option for emerging new aircraft thermal management applications. Although vapor-compression technology offers a steady state solution to many of the limitations of existing aircraft thermal management systems, industry concerns about transient behavior need to be addressed. The purpose of this research was to investigate transient effects on the vapor compression system when the majority of the onboard thermal loads are cooled directly with the vapor-compression system and how these systems operate under the rapid thermal transients that a military aircraft experiences during combat missions.

Hydrogen has difficulties in handling in a fuel cell vehicle, and has a fault with taking a big space there. The authors have proposed a hydrogen generation system using ammonia as a liquid fuel for fuel-cell electric vehicles. Ammonia has an advantage not to emit greenhouse effect gases because it does not contain a carbon atom. Hydrogen content of ammonia is 17.6 wt% and hydrogen quantity per unit mass is large. Ammonia can be easily dissociated to hydrogen and nitrogen by heating. Therefore, ammonia is an attractive hydrogen supply source for fuel cell vehicles. The ammonia hydrogen generation system of this study consists of a vaporizer, a heat exchanger and a cracking reactor with a separator. Ammonia is heated with the heat exchanger and sent to the cracking reactor, after it is evaporated through the vaporizer from the liquid ammonia. The ammonia is cracked to hydrogen and nitrogen with an appropriate catalyst.

A significant advantage of utilizing forced convection boiling heat transfer for spacecraft thermal management applications is that high heat fluxes are yielded that allow heat exchange equipment to be substantially compacted and reduced in mass. To date, two-phase heat transfer has not been utilized aboard spacecraft due to the uncertainty of the influence of gravity on the fluid dynamics and heat transfer within the heat exchange equipment. A subcooled flow boiling regime has recently been identified in which the bubble dynamics and two-phase flow controlling the heat transfer are independent of the gravitational field. The parametric bounds of this regime have been explored using a numerical simulation of the bubble dynamics in subcooled forced convection boiling. A gravity-independent/dependent regime map has been constructed, which depends on the Jacob number, Reynolds number, Weber number, and a liquid/vapor density ratio.

The thermal management of modern aircraft has become more challenging as aircraft capabilities have increased. The use of thermally resistant composite skins and the desire for low observability, reduced ram inlet size and number, have reduced the ability to transfer heat generated by the aircraft to the environment. As the ability to remove heat from modern aircraft has decreased, the heat loads associated with the aircraft have increased. Early in the aircraft design cycle uncertainty exists in both aircraft requirements and simulation predictions. In order to mitigate the uncertainty, it is advantageous to design thermal management systems that are insensitive to design cycle uncertainty. The risk associated with design uncertainty can be reduced through robust optimization. In the robust optimization of the thermal management system, three noise factors were selected: 1) engine fan air temperature, 2) avionics thermal load, and 3) engine thrust.

Although computational fluid dynamics (CFD) simulations have been widely used to successfully resolve turbulence and boundary layer phenomena induced by microscale flow passages in advanced heat exchanger concepts, the expense of such simulations precludes their use within system-level models. However, the effect of component design changes on systems must be better understood in order to optimize designs with little thermal margin, and CFD simulations greatly enhance this understanding. A method is presented to introduce high resolution, 3-D conjugate CFD calculations of candidate heat exchanger cores into dynamic aerospace subsystem models. The significant parameters guiding performance of these heat exchangers are identified and a database of CFD solutions is built to capture steady and unsteady performance of microstructured heat exchanger cores as a function of the identified parameters and flow conditions.

One of the key challenges faced by every engineer in the electrical vehicle development is to get the maximum range out of a single battery charge. There are several factors that impact the vehicle's range like the ambient temperature, electrical load, cooling system's efficiency, etc. Increasing only the vehicle range poses problems with another important vehicle attribute which is the passenger comfort. So, it is imperative for the engineer to understand the trade-off between comfort and range so that he chooses the right control strategy. One of the important factors in affecting the vehicle range is the ambient temperature. At low temperatures, electrical power from the battery is used to warm up the cabin to improve the passenger comfort, thus discharging the battery very quickly. On the contrary, when the ambient temperature is high, it is important to cool batteries, electronics and electric motors to keep their efficiency high.

Recent turbine engine numerical modeling developments have significantly improved the capability to accomplish integrated system-level analyses of aircraft thermal, power, propulsion, and vehicle systems. Combining desired aircraft performance with thermal management challenges of modern aircraft, which include increased heat loads from components such as avionics and more-electric accessories, as well as maintaining engine components at specified operating temperatures, demands we look for solutions that maximize heat sink capacity while minimizing adverse impacts on engine and aircraft performance. Development of optimized aircraft thermal management architectures requires the capability to directly analyze the impact of thermal management components, such as heat exchangers, on engine performance. This paper presents a process to evaluate the impact of heat exchanger design and performance characteristics (e.g., volume and pressure drops) on engine performance.

In this work we present our recent effort in developing a novel heat exchanger based on endothermic chemical reaction (HEX reactor). The proposed HEX reactor is designed to provide additional heat sink capability for aircraft thermal management systems. Ammonium carbamate (AC) which has a decomposition enthalpy of 1.8 MJ/kg is suspended in propylene glycol and used as the heat exchanger working fluid. The decomposition temperature of AC is pressure dependent (60°C at 1 atmosphere; lower temperatures at lower pressures) and as the heat load on the HEX increases and the glycol temperature reaches AC decomposition temperature, AC decomposes and isothermally absorbs energy from the glycol. The reaction, and therefore the heat transfer rate, is controlled by regulating the pressure within the reactor side of the heat exchanger. The experiment is designed to demonstrate continuous replenishment of AC.

NASA is extending human exploration of space beyond the low earth orbit and moon to Mars. To save cost, it has been determined that In Situ Propellant Production (ISPP) is a key enabling technology in Mission to Mars. A cryocooler is needed to liquefy and store oxygen and methane on the Mars surface. In an earlier study by the authors, a single-stage reverse Brayton cycle cryocooler was proposed with neon as the working fluid. The cryocooler operates between 80K and 310K. It was shown that a highly effective recuperative heat exchanger is vital to the overall efficiency of the cryogenic system. To achieve a COP of 0.2 or better, the heat exchanger should have an effectiveness of 0.97 or better while the percentage pressure drop should be less than 3%. In this paper, the design and analysis of a highly effective micro heat exchanger is presented. The heat exchanger is a multi-layer pile of parallel square ducts. The cold and hot fluids flow in a counter flow manner.

The applications of high-power diode laser arrays are increasingly demanded in the consumer, medical and defense sectors. Careful attention must be given to the thermal control of these semiconductor devices. Each diode emits a modest amount of heat over the microscopic footprint of the device, which drives the heat flux density to extreme values on the order of kW/cm2. Current liquid heat exchangers are inefficient and will soon reach their limits for removing the heat as the optical power of diode lasers increase. Spray cooling offers an attractive thermal management solution for laser diodes. Spray cooling will also improve operating efficiency through precise temperature control.

Two recent studies highlight the unique cooling requirements of Directed Energy Weapons (DEW) and identify methods to address these requirements. Both systems generate substantial heat loads, one requiring more than 1 MW of cooling. Furthermore, much of the heat is generated within a small volume, resulting in a high heat flux. Both spray cooling with ammonia and microchannel heat exchangers with de-ionized water or ammonia were considered. In each case it was determined that the ultimate heat sink would be the ambient air. In one study the heat transfer process was more challenging due to a relatively narrow allowable temperature range and a maximum allowable temperature near the ambient air temperature. Heat transfer options considered the use of a liquid loop with either direct ram air cooling, an air cycle cooling system, and a vapor cycle cooling system.