Increasing Heat Load Research Articles

The Droplets Condensate Centering in the Vapour Channel of Short Low Temperature Heat Pipes at High Heat Loads

The results of experimental studies of the process of condensate microdroplets centering contained in the moving moist vapour in the vapour channel of short heat pipes (HPs) for large thermal loads are presented. A vapour channel formed by capillary-porous insert in the form of the inner Laval-liked nozzle along the entire length of the HP. In the upper cover forming a condensation surface in the HP, on the diametrical line are installed capacitive sensors, forming three capacitors located at different distances from the longitudinal axis of the vapour channel. With increasing heat load and the boil beginning in the evaporator a large amount of moist vapour in the vapour channel of HP occur the pressure pulsation with frequency of 400-500 Hz and amplitude up to 1·104Pa. These pulsations affect the moving of the inertial droplets subsystem of the vapour and due to the heterogeneity of the velocity profile around the particle flow in the vapour channel at the diameter of microdroplets occurs transverse force, called the Saffman force and shear microdroplets to the center of vapour channel. Using installed in the top cover capacitors we can record the radial displacement of the condensable microdroplets.

Air adsorption on the gas-liquid interface in vapor condensation across horizontal tube

Air adsorption on the gas-liquid interface in vapor condensation across horizontal tube was investigated experimentally. An air hole with a diameter of 0.006m was opened at z=0.755m and θ = 50° on the jacket tube wall. A total of 16 thermistors were mounted uniformly on the horizontal tube wall along the circumference of z=0.27m and z=0.72m to measure wall temperature. The relative air content, ending difference, heat transfer coefficient and Nusselt number were calculated, and the in-leakage air adsorption process on the gas-liquid interface was analysed. The key factors that affect air adsorption on the gas-liquid interface are relative air content, heat load, pressure, and temperature. The air adsorption amount on the gas-liquid interface improves with the increase in relative air content and reduces with the increase in heat load. The air adsorption on the gas-liquid interface in the condensation of vapor over the horizontal tube has a similar change rule as Langmuir’s isothermal adsorption law. The air adsorption amount on the gas-liquid interface increases with the increase in pressure and the reduction in temperature. Higher vapor condensation decreases the air adsorption amount on the gas-liquid interface, causing the adsorption equilibrium to move towards desorption. The error between the variation of experimental in-leakage air mass flux with the pressure drop rate and the results from a formula recommended by America Heat Exchange Institute (HEI) is within −28%–23%. The present experimental results were compared with the results of the works of Nusselt (1916), Sherkriladze and Gomelauri (1966), Rose (1984), Fujii et al. (1972) and Chen and Lin (2009).

ОХОЛОДЖЕННЯ ПОВІТРЯ НА ВХОДІ СУДНОВОГО ДИЗЕЛЯ ЕЖЕКТОРНОЮ ХОЛОДИЛЬНОЮ МАШИНОЮ З БУСТЕРНИМ ТЕПЛОВИМ НАСОСОМ

Cooling of intake air for the main engine of transport ship by ejector chiller utilizing the heat of exhaust gases has been analyzed. The necessity to increase a potential of the heat recovered during increased heat loads upon engine intake air cooling system at definite parts of the route line was shown for the rout line "Mariupol-Amsterdam-Mariupol" as an example. An application of heat pump for heating purposes by utilizing the heat of exhaust condensate that is conventionally taken away by sea water through condensate cooler before the heat box has been proposed. The corresponding schemes of the engine intake air cooling system by ejector chiller with buster heat pump were developed.

ПІДВИЩЕННЯ ЕФЕКТИВНОСТІ ВИКОРИСТАННЯ АБСОРБЦІЙНОЇ БРОМИСТОЛІТІЄВОЇ ХОЛОДИЛЬНОЇ МАШИНИ В КОМБІНОВАНІЙ СИСТЕМІ ОХОЛОДЖЕННЯ ПОВІТРЯ НА ВХОДІ ГАЗОТУРБІННОЇ УСТАНОВКИ

The results of calculation of two stage gas turbine unit intake air cooling processes with previous cooling by absorption lithium-bromide chiller and deep cooling by ejector chiller are presented. The efficiency of the use of absorption lithium-bromide chiller cooling potential for changeable heat loads is analyzed. A scheme of cooling system with accumulating of surplus absorption lithium-bromide chiller cooling potential during decreased heat loads and its spending at increased heat loads has been proposed.

Experimental study on thermal performance of an anti-gravity pulsating heat pipe and its application on heat recovery utilization

A series of experimental studies are conducted to investigate the thermal performance of a pulsating heat pipe (PHP) under anti-gravity operation. The working fluid in the PHP is methanol and the Bond number of these experiments ranges from 0.833 to 1.068. Via the comparison between the performance of the anti-gravity PHP and a gravity-assisted one under different working conditions, the effects of gravity, heat load, filling ratio, number of turns and length of each section on the thermal performance of the anti-gravity PHP are discussed and analyzed. The experimental results indicate that an anti-gravity PHP always experiences a longer start-up process, exhibits more intense quasi-steady temperature oscillations and has a greater thermal resistance, implying that gravity plays an important positive role in the thermal performance of the PHP. For a given heat load, increasing the number of turns and the length of each section of the anti-gravity PHP can both lead to more intense temperature oscillation among the PHP but result in better and worse thermal performance of the PHP, respectively. The optimum filling ratio for the heat transfer performance of the anti-gravity pulsating heat pipe is approximately 70%. In addition, with the increasing heat load, the effect of other operational parameters on the thermal performance of the anti-gravity PHP can be weakened. Accordingly, a conceptual apparatus for utilization of the anti-gravity PHP for waste heat recovery from the high-temperature exhaust to preheat the low-temperature fuel is developed and tested with the Bond number of 0.902. Compared to the conventional pure copper heat transfer medium, the anti-gravity PHP shows much better heat recovery capacity, which verifies the great potential of an anti-gravity PHP for heat recovery utilization.

Simulation and performance analysis of combined parallel solar thermal and ground or air source heat pump systems

The combination of solar thermal (ST) systems and heat pumps (HP) is a promising hybrid heating system concept for efficient energy supply of buildings. In this work, a simulation study on the performance of combined ST and HP systems and a comparison with systems without ST collectors for different types of buildings and climates is presented. With regard to the dominating market-available systems, the evaluation is limited on systems with parallel integration of ST collectors. For a better transparency of the results, the simulation models and the main model parameterization are explained in detail. The results of the simulations show that the performance of HP systems can significantly be increased by adding a ST system. The relative increase of the performance decreases with increasing space heating demand of the building and is lower for climates with low ambient temperatures. In contrast, in most cases the absolute electricity savings increase with increasing space heating demand and are higher for systems with air source HPs. The seasonal performance factor (SPF) of parallel solar thermal and ground source heat pump (SGSHP-P) systems is between 0.5–1.1 (Strasbourg) and 1.0–2.0 (Helsinki) higher than the SPF of parallel solar thermal and air source heat pump (SASHP-P) systems with the same ST collector area. Hence, the difference between the performance of SASHP-P and SGSHP-P systems increases with increasing heat load and decreasing ambient temperatures of the location. However, in case of moderate climates like Strasbourg, SASHP-P systems can achieve the same or even higher values of SPF than ground source heat pump (GSHP) systems without ST collectors. Especially for buildings with low energy demand for space heating, the SPF is higher (up to 78.5% with a collector area of 25m2) and the systems can compete with each other. In a nutshell, in particular if the investment costs of a borehole heat exchanger and a ST system are similar or if there is no possibility to use a GSHP system with borehole heat exchanger, using a SASHP-P system may be a useful opportunity for a heating system concept with high energy performance and low electricity demand.

Operational characteristics of loop heat pipes with porous copper fiber sintered sheet as wick

Porous copper fiber sintered sheets (PCFSS) as wick are fabricated for loop heat pipes (LHP) using low-temperature solid-phase sintering technology and smooth and rough copper fiber as the manufacturing material. The operational characteristics of LHP with varied wick surface morphologies and filling ratios are analyzed. This study focuses on evaluating the following characteristics: thermal resistance characteristics under increasing and decreasing heat load patterns, evaporator surface thermal homogeneity, thermal inertia, and limit operational characteristics. Experimental results demonstrated that LHP with rough PCFSS exhibited lower thermal resistance. As compared to decreasing heat load conditions, relatively lower thermal resistance of LHP under increasing heat load conditions was obtained. The evaporator surface thermal homogeneity yielded improvement with rough PCFSS and increased filling ratio. A larger thermal inertia of LHP was observed when the low filling ratio was applied, and it will be improved when the filling ratio was increased and the smaller heat load variation intensity was selected. When rough PCFSS wick with 70% porosity and a deionized water filling ratio of 30% were selected, the LHP was able to effectively operate under 5–200W heat load conditions and yielded prompt response to heat load variation.

Unsteady heat transfer performance of heat pipe with axially swallow-tailed microgrooves

A mathematical model is developed for predicting the transient heat transfer and fluid flow of heat pipe with axially swallow-tailed microgrooves. The effects of liquid convective heat transfer in the microgrooves, liquid-vapor interfacial phase-change heat transfer and liquid-vapor interfacial shear stress are accounted for in the present model. The coupled non-linear control equations are solved numerically. Mass flow rate at the interface is obtained from the application of kinetic theory. Time variation of wall temperature is studied from the initial startup to steady state. The numerical results are verified by experiments. Time constants for startup and shutdown operation are defined to determine how fast a heat pipe responds to an applied input heat flux, which slightly decreases with increasing heat load.

A 63 K phase change unit integrating with pulse tube cryocoolers

This article presents the design and computer model results of an integrated cooler system which consists of a single stage pulse tube cryocooler integrated with a small amount of a phase change material. A cryogenic thermal switch was used to thermally connect the phase change unit to the cold end of the cryocooler. During heat load operation, the cryogenic thermal switch is turned off to avoid vibrations. The phase change unit absorbs heat loads by melting a substance in a constant pressure-temperature-volume process. Once the substance has been melted, the cryogenic thermal turned on, the cryocooler can then refreeze the material. Advantages of this type of cooler are no vibrations during sensor operations; the ability to absorb increased heat loads; potentially longer system lifetime; and a lower mass, volume and cost. A numerical model was constructed from derived thermodynamic relationships for the cooling/heating and freezing/melting processes.

Topography-associated thermal gradient predicts warming effects on woody plant structural diversity in a subtropical forest

Understanding global warming effects on forest ecosystems will help policy-makers and forest managers design forest management and biodiversity conservation strategies. We examined the change in woody plant structural diversity in response to topography-associated thermal gradients in a subtropical forest with diverse abundance patterns. We found that energy distribution in a warming trend across slopes had significant effects on woody plant structural diversity. Except for total basal area of the adult trees, plant structural diversity significantly decreased with the increase of heat load. Heat load is significantly and negatively correlated with number of stems, number of species, and the number of stems of the most abundant species (Nmax) for seedlings, saplings, and individuals of all sizes. For the adult trees, heat load is significantly and positively correlated with number of stems and Nmax, and negatively but not significantly with number of species, indicating that large trees may not be as sensitive as seedlings and saplings to warming. Partial correlation analysis, having controlled for elevation, strengthened those relations in most cases. Our results reveal that warming will increase community productivity by enhancing the growth of large trees, but decrease species diversity and inhibit the regeneration of tree seedlings and saplings.

Numerical investigation of heat transfer in thermosyphon under the emergency mode of operation of lithium-ion batteries of aircraft

Numerical study of heat transfer in a closed two-phase thermosyphon using the software package ANSYS FLUENT is carried out. Characteristic temperature distributions, current lines and velocity vectors are obtained at critical thermal loads. The temperature difference in the investigated region decreases with increasing heat load was obtained.

Self pumping magnetic cooling

Efficient thermal management and heat recovery devices are of high technological significance for innovative energy conservation solutions. We describe a study of a self-pumping magnetic cooling device, which does not require external energy input, employing Mn–Zn ferrite nanoparticles suspended in water. The device performance depends strongly on magnetic field strength, nanoparticle content in the fluid and heat load temperature. Cooling (ΔT) by ~20 °C and ~28 °C was achieved by the application of 0.3 T magnetic field when the initial temperature of the heat load was 64 °C and 87 °C, respectively. These experiments results were in good agreement with simulations performed with COMSOL Multiphysics. Our system is a self-regulating device; as the heat load increases, the magnetization of the ferrofluid decreases; leading to an increase in the fluid velocity and consequently, faster heat transfer from the heat source to the heat sink.

CFD Analysis of Different Cooling Options for a Gyrotron Cavity

Three different pressurized subcooled water cooling options [hypervapotron, mini-channels (MCs), and meander flow drivers] for a full-size gyrotron cavity are developed and analyzed using the commercial computational fluid dynamics code STAR-CCM+, to guarantee a pressure drop below the maximum acceptable, and the compliance with manufacturing constraints. The full-size designs are then transposed to corresponding mock-ups to identify the most suitable candidate for an experimental test. The conjugate multiphase heat transfer problem is solved in the three mock-up geometries, aimed at comparatively assessing their thermal performance at increasing heat load on the target up to 24 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The most promising option for an effective cooling of the cavity results to be the mini-channels.

Experimental Assessment of a Domestic Split Type Air-Conditioner Working with R410A

The objective of the present work is to assess experimentally the performance characteristics of a R410A domestic air conditioner under different in-door operating conditions. In order to achieve this objective, a test facility of the investigated system is developed and experiments are conducted. Experimental results on R410A air conditioning system are obtained over a wide range of in-door operating conditions. Experimental results confirmed that the cooling capacity, compressor power and COP increase by 90.9%, 5.2% and 81.5%, respectively, as air humidity ratio increases from 8 to 25.5[Formula: see text]gw kg[Formula: see text]. The cooling capacity, condenser heat load, compressor power and COP increase by 7.2%, 6.1%, 2.8% and 4.1%, respectively, when the evaporator air inlet temperature increases from 28[Formula: see text]C to 34[Formula: see text]C for a given humidity ratio. COP increases by 25.6% while pressure ratio decreases by 2.5% as the air volume flow rate increases from 400 to 550[Formula: see text]m3 h[Formula: see text].

High-Speed Visual Analysis of Fluid Flow and Heat Transfer in Oscillating Heat Pipes with Different Diameters

The oscillating heat pipe (OHP) is a new member in the family of heat pipes, and it has great potential applications in energy conservation. However, the fluid flow and heat transfer in the OHP as well as the fundamental effects of inner diameter on them have not been fully understood, which are essential to the design and optimization of the OHP in real applications. Therefore, by combining the high-speed visualization method and infrared thermal imaging technique, the fluid flow and thermal performance in the OHPs with inner diameters of 1, 2 and 3 mm are presented and analyzed. The results indicate that three fluid flow motions, including small oscillation, bulk oscillation and circulation, coexist or, respectively, exist alone with the increasing heating load under different inner diameters, with three flow patterns occurring in the OHPs, viz. bubbly flow, slug flow and annular flow. These fluid flow motions are closely correlated with the heat and mass transfer performance in the OHPs, which can be reflected by the characteristics of infrared thermal images of condensers. The decrease in the inner diameter increases the frictional flow resistance and capillary instability while restricting the nucleate boiling in OHPs, which leads to a smaller proportion of bubbly flow, a larger proportion of short slug flow, a poorer thermal performance, and easier dry-out of working fluid. In addition, when compared with the 2 mm OHP, the increasing role of gravity induces the thermosyphon effect and weakens the ‘bubble pumping’ action, which results in a little smaller and bigger thermal resistances of 3 mm OHP under small and bulk oscillation of working fluid, respectively.

Energy and Exergy Analysis of Wood-based CHP. Case Study

The paper presents use of energy and exergy analyses for the assessment of input and output flows in a cogeneration plant, shows points of exergy degradation in a system and allows to make a quantitative evaluation of energy quality changes. The objective of the study is to determine the essential points and values of exergy destruction in a biomass combined heat and power (CHP) plant in Jelgava, as well as to define boundaries of the method's use. A mathematical model is elaborated, used and evaluated as suitable for its application for the real plant and may be used in the plant's further operation for efficiency assessment and improvements. The results are shown with Sankey and Grassmann diagrams; the largest exergy destruction occurs in the boiler (steam generator) and represents 63.7% of input fuel exergy; second largest exergy destruction is observed in heat exchangers of district heating (DH) system. Exergy efficiency of the plant is influenced by the power to heat ratio (α) and potential of the produced heat (temperature). Increased heat load or decreased temperature of heat delivered to the consumer lowers the value of exergy efficiency of the plant.

Patterns of façade system design for enhanced energy performance of multistory buildings

The paper presents a study of the effect of geometric design of equatorial-facing, double skin façade on energy performance of multi-story office buildings. The investigated parameters relate to the outer skin surface geometry and to the associated air cavity depth between the two skins of this façade system. Two main geometrical designs are investigated. A simple folded plate geometry – saw-tooth that consists of a single fold, and a more complex folded-plate units based on pyramids.The results indicate that although deviation from the basic flat façade leads generally to an increase in heating load, this is counterbalanced by a reduction in cooling load, and a considerable increase in energy generation potential from façade integrated photovoltaic systems. The position of fold and the depth of the cavity have a significant effect on thermal load and energy generation potential. Under the studied climatic conditions (Calgary, Alberta, Canada – 52°N), The total annual electricity generation potential, by the multifold configurations can exceed that of the flat façade by up to 80%. The increase in total electricity generation potential of some designed configurations is accompanied by an increase in time spread of functional electricity over extended period of time. Multifold façade configurations that combine east and west orientations can provide a peak spread of some 4–5hours.

Enhancement of urban heat load through social inequalities on an example of a fictional city King\u2019s Landing

The numerical model MUKLIMO_3 is used to simulate the urban climate of an imaginary city as an illustrative example to demonstrate that the residential areas with deprived socio-economic conditions can exhibit an enhanced heat load at night, and thus more disadvantageous environmental conditions, compared with the areas of higher socio-economic status. The urban climate modelling simulations differentiate between orographic, natural landscape, building and social effects, where social differences are introduced by selection of location, building type and amount of vegetation. The model results show that the increase of heat load can be found in the areas inhabited by the poor population as a combined effect of natural and anthropogenic factors. The unfavourable location in the city and the building type, consisting of high density, low housing with high fraction of pavement and small amount of vegetation contribute to the formation of excessive heat load. This abstract example shows that the enhancement of urban heat load can be linked to the concept of a socially stratified city and is independent of the historical development of any specific city.

Comparative assessment on irreversible losses in heat pumps using R744/R32/R1234yf and R744/R32/R1234ze(E)

This article presents an experimental assessment of coefficient of performance and irreversible loss in the heat pump cycle using hydro-fluoro-olefin base ternary mixtures: YF300 (R744/R32/R1234yf; 4/44/52 mass%), YF200 (R744/R32/R1234yf; 5/28/67 mass%), ZE300 (R744/R32/R1234ze(E); 4/43/53 mass%), and ZE200 (R744/R32/R1234ze(E); 9/29/62 mass%). The smaller volumetric capacity of YF200 and ZE200 increases the irreversible loss caused by the pressure drop with increasing heat load. ZE200, which has a temperature glide of approximately 22 K, increases the irreversible loss in the condenser, evaporator, and expansion valve. The experimental results indicate that YF300 and ZE300 could be potential alternatives to R410A and R32.

Heat transfer performance of a novel double-layer mini-channel heat sink

High pressure drop and significant non-uniformity in temperature distribution along the streamwise direction are still challenges to the design of mini-channel heat sink. High density mini-channel arrays with high liquid-wall contact area are usually pursued in a conventional single-layer design of heat sink, which also inevitably brings high pressure drop. A novel double-layer structured heat sink is proposed in this paper. Four heat sinks with various designs in mini-channel density and flow direction were fabricated and studied experimentally on the heat transfer performance. The single factor of heat load does not show obvious effect on the overall thermal resistance of the heat sinks. On the other hand, slight decrease in thermal resistance was found with the increase in heat load at high flow rates. Moreover, a computational fluid dynamics modeling work was conducted. The results indicate that the parallel cross-flow field regulated by the double-layer structure enhances the heat exchange in both horizontal and vertical directions and consequently gives an uniform temperature distribution and high heat transfer efficiency.