Showing papers on "Heat exchanger published in 2013"

Thermometry, calorimetry, and mean body temperature during heat stress.

Abstract: Heat balance in humans is maintained at near constant levels through the adjustment of physiological mechanisms that attain a balance between the heat produced within the body and the heat lost to the environment. Heat balance is easily disturbed during changes in metabolic heat production due to physical activity and/or exposure to a warmer environment. Under such conditions, elevations of skin blood flow and sweating occur via a hypothalamic negative feedback loop to maintain an enhanced rate of dry and evaporative heat loss. Body heat storage and changes in core temperature are a direct result of a thermal imbalance between the rate of heat production and the rate of total heat dissipation to the surrounding environment. The derivation of the change in body heat content is of fundamental importance to the physiologist assessing the exposure of the human body to environmental conditions that result in thermal imbalance. It is generally accepted that the concurrent measurement of the total heat generated by the body and the total heat dissipated to the ambient environment is the most accurate means whereby the change in body heat content can be attained. However, in the absence of calorimetric methods, thermometry is often used to estimate the change in body heat content. This review examines heat exchange during challenges to heat balance associated with progressive elevations in environmental heat load and metabolic rate during exercise. Further, we evaluate the physiological responses associated with heat stress and discuss the thermal and nonthermal influences on the body's ability to dissipate heat from a heat balance perspective.

Thermal Cracking of Hydrocarbon Aviation Fuels in Regenerative Cooling Microchannels

Abstract: Regenerative cooling with hydrocarbon aviation fuels on board is taken as a promising technology for the thermal management system of next-generation aircraft. An improved methodology of an electrically heated tube (1 mm i.d.), i.e., applying the variable reactor tube length to carry on thermal cracking of supercritical hydrocarbon aviation fuels as the electric current heating maintains constant, was proposed to experimentally obtain detailed information on the local concentration and temperature along the microchannels of a heat exchanger. For the first time a series of experimental data on detailed local chemical compositions of cracked hydrocarbon fuel along the cooling microchannels were reported under supercritical conditions (5 MPa, 680–700 °C), and the calculated thermodynamic properties, velocity, and residence times along the tube were also reported. A modified molecular reaction model consisting of 18 species and 24 reactions was developed to predict thermal cracking of hydrocarbon aviation fue...

A Comparison of Supercritical Carbon Dioxide Power Cycle Configurations with an Emphasis on CSP Applications

Abstract: Recent research suggests that an emerging power cycle technology using supercritical carbon dioxide (s-CO2) operated in a closed-loop Brayton cycle offers the potential of equivalent or higher cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for CSP applications. Preliminary design-point modeling suggests that s-CO2 cycle configurations can be devised that have similar overall efficiency but different temperature and/or pressure characteristics. This paper employs a more detailed heat exchanger model than previous work to compare the recompression and partial cooling cycles, two cycles with high design-point efficiencies, and illustrates the potential advantages of the latter. Integration of the cycles into CSP systems is studied, with a focus on sensible heat thermal storage and direct s-CO2 receivers. Results show the partial cooling cycle may offer a larger temperature difference across the primary heat exchanger, thereby potentially reducing heat exchanger cost and improving CSP receiver efficiency.

Experimental and analytical studies of earth–air heat exchanger (EAHE) systems in India: A review

Abstract: This is the property of earth that at a depth of about 1.5 to 2 m, the temperature of ground remains almost constant throughout the year. This constant temperature is called earth’s undisturbed temperature which remains higher than surface temperature of earth in winter season and vice versa in summer. For effective utilization of heat capacity of earth, the earth–air heat exchanger (EAHE) has to be designed. The EAHEs are considered as an effective passive heating/cooling medium for buildings. This is basically a series of metallic, plastic or concrete pipes buried underground at a particular depth through which the fresh atmospheric air flows and gets heated in winter and supplied to the building if at sufficiently high temperature and vice versa in summer. Till date many researchers have carried out a number of studies in designing, modeling and testing of EAHEs systems. This paper reviews on the experimental and analytical studies of EAHE systems around the world but the studies are mainly focused on EAHE systems at the Indian universities as of the end June, 2012.

System and method for high efficiency power generation using a carbon dioxide circulating working fluid

Abstract: The present invention provides methods and system for power generation using a high efficiency combustor in combination with a CO 2 circulating fluid. The methods and systems advantageously can make use of a low pressure ratio power turbine and an economizer heat exchanger in specific embodiments. Additional low grade heat from an external source can be used to provide part of an amount of heat needed for heating the recycle CO 2 circulating fluid. Fuel derived CO 2 can be captured and delivered at pipeline pressure. Other impurities can be captured.

Experimental analysis of R1234yf as a drop-in replacement for R134a in a vapor compression system

Abstract: This paper presents an experimental analysis of a vapor compression system using R1234yf as a drop-in replacement for R134a. In this work, we compare the energy performance of both refrigerants, R134a and R1234yf, in a monitored vapor compression system under a wide range of working conditions. So, the experimental tests are carried out varying the condensing temperature, the evaporating temperature, the superheating degree, the compressor speed, and the internal heat exchanger use. Comparisons are made taking refrigerant R134a as baseline, and the results show that the cooling capacity obtained with R1234yf in a R134a vapor compression system is about 9% lower than that obtained with R134a in the studied range. Also, when using R1234yf, the system shows values of COP about 19% lower than those obtained using R134a, being the minor difference for higher condensing temperatures. Finally, using an internal heat exchanger these differences in the energy performance are significantly reduced.

Topology optimization for fluid---thermal interaction problems under constant input power

Abstract: This paper deals with density-based topology optimization considering fluid and thermal interactions, in which the Navier---Stokes and heat transport equations are coupled. We particularly focus on designing heat exchangers. In the engineering context, heat exchangers are designed while considering a certain amount of input power. Therefore it is important to maximize the performance of a heat exchanger under a constant input power. In this paper we propose a way to control the input power by introducing an extra integral equation. To be more precise, in the fluid analysis, the inlet pressure is determined by solving the extra integral equation together with the Navier---Stokes equation. By doing this we can keep the inlet power constant even when the flow channels are changed in the optimization process. Consequently, the system of equations of the fluid field takes an integrodifferential form. On the other hand, in the heat transport analysis, a single governing equation is defined for simultaneously modeling both the solid and fluid parts. The design variable is a fluid fraction whose distribution represents the topology of the solid and fluid domains. When designing heat exchangers, two different heat conditions are considered in the formulation of the optimization problems, namely temperature-dependent and temperature-independent heat sources. Through the numerical examples for designing flow channels in a heat exchanger, it is shown that distinct topologies can be obtained according to the input power and the heat source conditions.

Thermoelectric Generators for Automotive Waste Heat Recovery Systems Part I: Numerical Modeling and Baseline Model Analysis

Abstract: A numerical model has been developed to simulate coupled thermal and electrical energy transfer processes in a thermoelectric generator (TEG) designed for automotive waste heat recovery systems. This model is capable of computing the overall heat transferred, the electrical power output, and the associated pressure drop for given inlet conditions of the exhaust gas and the available TEG volume. Multiple-filled skutterudites and conventional bismuth telluride are considered for thermoelectric modules (TEMs) for conversion of waste heat from exhaust into usable electrical power. Heat transfer between the hot exhaust gas and the hot side of the TEMs is enhanced with the use of a plate-fin heat exchanger integrated within the TEG and using liquid coolant on the cold side. The TEG is discretized along the exhaust flow direction using a finite-volume method. Each control volume is modeled as a thermal resistance network which consists of integrated submodels including a heat exchanger and a thermoelectric device. The pressure drop along the TEG is calculated using standard pressure loss correlations and viscous drag models. The model is validated to preserve global energy balances and is applied to analyze a prototype TEG with data provided by General Motors. Detailed results are provided for local and global heat transfer and electric power generation. In the companion paper, the model is then applied to consider various TEG topologies using skutterudite and bismuth telluride TEMs.

Vehicle air conditioner

Abstract: [Problem] To provide a vehicle air conditioner that can reduce electricity consumption by improving the accuracy of frost determination on an outdoor heat exchanger, maintaining heating performance, and avoiding unnecessary defrosting. [Solution] A controller that implements a heating mode. A controller (32) ascertains the formation of frost on an outdoor heat exchanger (7) either on the basis of the refrigerant evaporation pressure (PXO) of the outdoor heat exchanger (7) and the refrigerant evaporation pressure (PXObase) of the outdoor heat exchanger (7) when no frost is present, or on the basis of the refrigerant evaporation temperature (TXO) of the outdoor heat exchanger (7) and the refrigerant evaporation temperature (TXObase) of the outdoor heat exchanger (7) when no frost is present, and then feeds high temperature refrigerant gas to the outdoor heat exchanger (7) to defrost the outdoor heat exchanger (7).

The design and environmental evaluation of earth-to-air heat exchangers (EAHE). A literature review

Abstract: There is a rising interest in heating and cooling systems based on renewable energy sources. Air heating or cooling with earth-to-air heat exchangers (EAHE) is one approach for reducing ventilation heat losses and improving thermal comfort in buildings. A literature research was performed in order to analyze the design, characteristics of earth-to-air heat exchangers and whether they could be coupled with HVAC system coupling. A range of projects was compared in order to collect and summarize design suggestions.

Air conditioning device

Abstract: PROBLEM TO BE SOLVED: To obtain an air conditioning device allowing dehumidification while supplying a sufficient air flow even when the cooling load is small.SOLUTION: The air conditioning device includes a compressor 1, an outdoor heat exchanger 3, electric expansion valves 4-6, and an indoor heat exchanger 8 which are sequentially connected by a refrigerant pipe 20 to form a refrigeration cycle. The indoor heat exchanger 8 includes a plurality of heat exchangers 8a, 8b, and these heat exchangers are connected to distribution flow paths 21, 22 diverging in parallel from the refrigerant pipe. The respective distribution flow paths are provided with the electric expansion valves 5, 6. The air conditioning device also includes a suction air temperature sensor 14 for detecting temperature of air flowing into the indoor heat exchanger, a discharge pressure sensor 11 for detecting discharge pressure of the compressor, and a suction pressure sensor 10 for detecting suction pressure. In addition, the air conditioning device includes a control means 13 for closing an electric expansion valve corresponding to any of the plurality of heat exchangers when a difference between the temperature detected by the suction air temperature sensor and set temperature is smaller than a predetermined value and when dehumidifying operation is performed during low-load operation.

Climate control system for hybrid vehicles using thermoelectric devices

Abstract: The present invention provides a system for controlling the climate of a hybrid vehicle. The system includes a thermoelectric module, a heat exchanger, a pump, and a valve. The thermoelectric module includes thermoelectric elements powered by electric energy. The thermoelectric elements emit or absorb heat energy based on the polarity of the electrical energy provided. A tube containing coolant runs proximate the thermoelectric elements. To aid in the transfer of heat energy, a blower is provided to generate an air flow across the thermoelectric elements and the tube. The coolant is provided from the thermoelectric module to a heat exchanger that heats or cools the air flow provided to the cabin of the vehicle. The pump and valve are in fluid communication with the heat exchanger and thermoelectric module. The pump pressurizes the coolant flow through the tube and coolant lines. In a cooling mode, the valve is configured to selectively bypass the engine coolant system of the vehicle.