Increasing Heat Load Research Articles

Pore Network Simulations of Heat and Mass Transfer inside an Unsaturated Capillary Porous Wick in the Dry-out Regime

In this work, a two-dimensional pore network model is developed to study the heat and mass transfer inside a capillary porous wick with opposite replenishment in the dry-out regime. The mass flow rate in each throat of the pore network is computed according to the Hagen–Poiseuille law, and the heat flux is calculated based on Fourier’s law with an effective local thermal conductivity. By coupling the heat and the mass transfer, a numerical method is devised to determine the evolution of the liquid–vapor interface. The model is verified by comparing the effective heat transfer coefficient versus heat load with experimental observations. For increasing heat load, an inflation/deflation of the vapor pocket is observed. The influences of microstructural properties on the vapor pocket pattern and on the effective heat transfer coefficient are discussed: A porous wick with a non-uniform or bimodal pore size distribution results in a larger heat transfer coefficient compared to a porous wick with a uniform pore size distribution. The heat and mass transfer efficiency of a porous wick comprised of two connected regions of small and large pores is also examined. The simulation results indicate that the introduction of a coarse layer with a suitable thickness strongly enhances the heat transfer coefficient.

Visual investigation on startup characteristics of a novel loop heat pipe

In this experiment, visual investigations of pool boiling, redistribution of working fluid and temperature measurements were carried out to study the startup characteristics of a novel loop heat pipe in different conditions. The mechanisms behind were also explored. It is found that the driven force in this loop was composed of capillary force and the vapor pressure formed in the boiling pool, and the final startup could be obtained only when the vapor pressure ensured the subcooled liquid ingress into the compensation chamber and capillary force ensured the liquid supply from the compensation chamber to the boiling pool. The typical startup process was made up of seven stages, among which a peculiar breathing phenomenon occurred in the compensation chamber. With increasing heat load of boiling pool, a quicker startup was obtained and the peak temperature of evaporator was reduced significantly. However, it caused a quite serious overheating problem of boiling pool. With increasing time difference, a more violent boiling phenomenon appeared in compensation chamber. The loop started up at a higher peak temperature of evaporator, while that of boiling pool remained unchanged. Moreover, in the condition of an insufficient liquid charging and a loose fit, a pulsation operation mode of the loop system was observed.

Effect of C60 nanofluid on the thermal performance of a flat-plate pulsating heat pipe

An experimental study on heat-transfer performance of a flat-plate closed loop pulsating heat pipe (FCLPHP) with C60 nanofluid was carried out. The C60 nanofluid was a mixture of ethanol and C60 nano-particles at three different concentrations of 0.1wt%, 0.2wt% and 0.3wt%. The experimental results show that increasing C60 nanofluid concentration improves the thermal performance of the FCLPHP, but reduces the critical heat load, at which FCLPHP begins to dry out. The critical heat load was reduced from 70W for base fluid to 35W for nanofluid with concentration of 0.3wt%. In order to evaluate the improvement of the heat transfer performance of the nanofluid FCLPHP, improving rate τ is used. The higher the τ value, the greater the degree of improvement is. It is found that τ firstly increases up to a maximum value and then decreases gradually with the increase of heat load. The maximum τ value was 36% when the heat load was 20W at a concentration of 0.3wt%. A regression model was proposed for the FCLPHP, with which a sensitivity analysis was made. The analysis shows that the heat load impacts the thermal performance more than the concentration does.

Electro-magnetic stress-induced degradation of insulation vacuum of a large cryo-magnetic system

In superconducting magnets, the cold mass is placed in a vacuum vessel to reduce heat load to the liquid helium system. Helium leaks into the vacuum vessel can degrade the insulation vacuum, which can, in turn, cause an increase in the heat load to the liquid helium system. These leaks are called cold leaks, as they show up when the coil is cooled with liquid helium. K500 superconducting cyclotron magnet at Variable Energy Cyclotron Centre, Kolkata has such cold leaks in the helium vessel that developed during cool down. The leak rate increases with the increase of current in the superconducting coils. This paper describes a series of experiments carried out on the superconducting cyclotron magnet to find the level of degradation of insulation vacuum and measure the increase in heat load with magnet current. The leak rate was also measured and the leak size was estimated analytically. Detail magneto-structural analysis was done using Finite Element Method (FEM) to identify highly stressed zones in the helium vessel and found out that highly stressed zones coincide with the weld zones. The magneto-structural stress was applied on an estimated size of single crack and found that crack tip stress could reach beyond elastic limit of the material. We can predict that the full design current may be unachievable in this situation. Mitigation of increased heat load was also done using an additional vacuum pump for the insulation vacuum space.

Experimental Investigation on the Impact of Distorted Signal on Synthetic Jet

Effective techniques for cooling electronic devices must deal with increasing heat loads associated with higher heat flux density. Many conventional cooling techniques like fan are reaching the limits of their effectiveness and shape. The novel method of heat transfer enhancement is synthetic jet. In this paper experimental results are presented. Synthetic jet actuator consist of STX 6.5 inch speaker installed in metacrylate chamber with circular orifice. The actuator was powered with signal from amplifier. The signal consist of basic sinusoidal function and THD noise added with some amplitude. The root mean square of signal voltage was constant 4V. The properties of synthetic jet were measured using constant temperature thermo-anemometer. Instantaneous velocity of air was measured in the orifice center and compared with input signal. Additional RMS and average velocity of air were measured. Measurement revealed that input signal of synthetic jet generator can contain some noise without effect on RMS and average velocity of air in the orifice. The THD less than 1% does not cause negative effect on synthetic jet fluid velocity.

Identification of sixteen peptides reflecting heat and/or storage induced processes by profiling of commercial milk samples

Peptide profiles of different drinking milk samples were examined to study how the peptide fingerprint of milk reflects processing conditions. The combination of a simple and fast method for peptide extraction using stage tips and MALDI–TOF–MS enabled the fast and easy generation and relative quantification of peptide fingerprints for high-temperature short-time (HTST), extended shelf life (ESL) and ultra-high temperature (UHT) milk of the same dairies. The relative quantity of 16 peptides changed as a function of increasing heat load. Additional heating experiments showed that among those, the intensity of peptide β-casein 196–209 (m/z 1460.9Da) was most heavily influenced by heat treatment indicating a putative marker peptide for milk processing conditions. Storage experiments with HTST- and UHT milk revealed that the differences between different types of milk samples were not only caused by the heating process. Relevant was also the proteolytic activity of enzymes during storage, which were differently influenced by the heat treatment. These results indicate that the peptide profile may be suitable to monitor processing as well as storage conditions of milk. SignificanceIn the present study, peptide profiling of different types of milk was carried out by MALDI–TOF–MS after stage-tip extraction and relative quantification using an internal reference peptide. Although MALDI–TOF–MS covers only part of the peptidome, the method is easy and quick and is, therefore, suited for routine analysis to address several aspects of food authenticity. Using this method, 16 native peptides were detected in milk that could be modulated by different industrial processes. Subsequent heating and storage experiments with pasteurized and UHT milk confirmed that these peptides are indeed related to the production or storage conditions of the respective products. Furthermore, the heating experiments revealed one peptide, namely the β-casein-derived sequence β-casein 196–209, which underwent particularly sensitive modulation by heat treatment. The present results indicate that the modulated peptides, and especially β-casein 196–209, may be suitable markers to monitor processing parameters for industrial milk production. Furthermore, the model experiments suggest mechanisms leading to the formation or degradation of peptides, which help to evaluate putative marker peptides.

Modeling heat stress under different environmental conditions

Renewed interest in heat stress effects on livestock productivity derives from climate change, which is expected to increase temperatures and the frequency of extreme weather events. This study aimed at evaluating the effect of temperature and humidity on milk production in highly selected dairy cattle populations across 3 European regions differing in climate and production systems to detect differences and similarities that can be used to optimize heat stress (HS) effect modeling. Milk, fat, and protein test day data from official milk recording for 1999 to 2010 in 4 Holstein populations located in the Walloon Region of Belgium (BEL), Luxembourg (LUX), Slovenia (SLO), and southern Spain (SPA) were merged with temperature and humidity data provided by the state meteorological agencies. After merging, the number of test day records/cows per trait ranged from 686,726/49,655 in SLO to 1,982,047/136,746 in BEL. Values for the daily average and maximum temperature-humidity index (THIavg and THImax) ranges for THIavg/THImax were largest in SLO (22–74/28–84) and shortest in SPA (39–76/46–83). Change point techniques were used to determine comfort thresholds, which differed across traits and climatic regions. Milk yield showed an inverted U-shaped pattern of response across the THI scale with a HS threshold around 73 THImax units. For fat and protein, thresholds were lower than for milk yield and were shifted around 6 THI units toward larger values in SPA compared with the other countries. Fat showed lower HS thresholds than protein traits in all countries. The traditional broken line model was compared with quadratic and cubic fits of the pattern of response in production to increasing heat loads. A cubic polynomial model allowing for individual variation in patterns of response and THIavg as heat load measure showed the best statistical features. Higher/lower producing animals showed less/more persistent production (quantity and quality) across the THI scale. The estimated correlations between comfort and THIavg values of 70 (which represents the upper end of the THIavg scale in BEL-LUX) were lower for BEL-LUX (0.70–0.80) than for SPA (0.83–0.85). Overall, animals producing in the more temperate climates and semi-extensive grazing systems of BEL and LUX showed HS at lower heat loads and more re-ranking across the THI scale than animals producing in the warmer climate and intensive indoor system of SPA.

Estimating Over Temperature and Its Duration in a Flat Plate With Sudden Changes in Heating and Cooling

When the operating condition of a gas turbine engine changes from one steady-state to another, the cooling must ensure that the solid's temperatures never exceed the maximum allowable throughout the transient process. Exceeding the maximum allowable temperature is possible even though cooling is increased to compensate for the increase in heating because there is a time lag in how the solid responds to changes in its convective heating and cooling environments. In this paper, a closed-form solution (referred to as the 1D model) is derived to estimate the over temperature and its duration in a flat plate subjected to sudden changes in heating and cooling rates. For a given change in heating rate, the 1D model can also be used to estimate the minimum cooling needed to ensure that the new steady-state temperature will not exceed the maximum allowable. In addition, this model can estimate the temperature the material must be cooled to before imposing a sudden increase in heat load to ensure no over temperature throughout the transient process. Comparisons with the exact solutions show the 1D model to be accurate within 0.1%. This 1D model was generalized for application to problems in multidimensions. The generalized model was used to estimate the duration of over temperature in a two-dimensional problem involving variable heat transfer coefficient (HTC) on the cooled side of a flat plate and provided results that match the exact solution within 5%.

Increased Heat Load Effects on the Thermal Performance of Single- and Two-Layered Microchannels with Varying Axial Length and Micro Pin-Fin Inserts

Increased Heat Load Effects on the Thermal Performance of Single- and Two-Layered Microchannels with Varying Axial Length and Micro Pin-Fin Inserts

Design, R&D and commissioning of EAST tungsten divertor

After commissioning in 2005, the EAST superconducting tokamak had been operated with its water cooled divertors for eight campaigns up to 2012, employing graphite as plasma facing material. With increase in heating power over 20 MW in recent years, the heat flux going to the divertors rises rapidly over 10 MW m−2 for steady state operation. To accommodate the rapid increasing heat load in EAST, the bolting graphite tile divertor must be upgraded. An ITER-like tungsten (W) divertor has been designed and developed; and firstly used for the upper divertor of EAST. The EAST upper W divertor is modular structure with 80 modules in total. Eighty sets of W/Cu plasma-facing components (PFC) with each set consisting of an outer vertical target (OVT), an inner vertical target (IVT) and a DOME, are attached to 80 stainless steel cassette bodies (CB) by pins. The monoblock W/Cu-PFCs have been developed for the strike points of both OVT and IVT, and the flat type W/Cu-PFCs for the DOME and the baffle parts of both OVT and IVT, employing so-called hot isostatic pressing (HIP) technology for tungsten to CuCrZr heat sink bonding, and electron beam welding for CuCrZr to CuCrZr and CuCrZr to other material bonding.Both monoblock and flat type PFC mockups passed high heat flux (HHF) testing by means of electron beam facilities. The 80 divertor modules were installed in EAST in 2014 and results of the first commissioning are presented in this paper.

Dynamic effect of thermal bridges on the energy performance of a low-rise residential building

The existence of thermal bridges in building envelopes affects the energy performance of buildings, their durability and occupants’ thermal comfort. Typically the effect of thermal bridges on the energy performance is taken into account by implementing an equivalent U-value in 1D whole building energy simulation program. This treatment accounts for the effect of thermal bridges on the overall thermal transmittance, while their thermal inertia effect is ignored. The presence of thermal bridges not only reduces the overall thermal resistance but also changes the dynamic thermal characteristics of the envelope.This paper investigates the dynamic effect of thermal bridges on the energy performance of residential buildings through simulations. A two-story residential building is used as a case study. Three methods, namely equivalent U-value method, equivalent wall method, and direct 2D/3D modeling method, are implemented in WUFI Plus, a whole building Heat, Air and Moisture (HAM) modeling program. Simulations are carried out for two climates with different insulation levels. Simulation results show that for the cold climate the annual heating load of this building with the inclusion of thermal bridges modeled using 3D dynamic method is 8–13% higher than that modeled using the equivalent U-value method, and 4–9% higher than that modeled using the equivalent wall method. With the increase of the insulation level, the percentage effect of thermal bridges on the heating load increases, while the difference among the three methods decreases. For the hot climate, simulation results show that the presence of thermal bridges increases the annual cooling load by 20%. Compared to the 3D dynamic method, the annual cooling load is underestimated by 17% using the equivalent U-value method and by 14% using the equivalent wall method, respectively.

Development of a Copper Heat Pipe with Axial Grooves Manufactured Using Wire Electrical Discharge Machining (Wire-EDM)

In this research, a heat pipe with grooves was experimentally analyzed for the application in thermal management of electronic packaging. The heat pipe was produced by a copper tube with an outer diameter of 9.45 mm, length of 205 mm, and capillary structure composed by axial grooves with average diameter of 220 μm. The grooves were manufactured using wire electrical discharge machining (wire-EDM). The working fluid used was de-ionized water. The condenser was cooled by air forced convection and the evaporator was heated using an electrical resistor. This heat pipe was tested horizontally to increasing heat loads varying from 5 to 15 W. The experimental results showed that the heat pipe worked successfully.

Operational characteristics of a loop heat pipe with a flat evaporator and two primary biporous wicks

Loop heat pipe (LHP) is an efficient heat transfer device with excellent performance. Its operation is based on the evaporation and condensation of the internal working fluid, and the compensation chamber temperature controls the loop operating temperature. Compared with cylindrical evaporator LHP, the flat evaporator LHP benefits from the flat thermo-contact surface, but suffers from more heat leak. In this paper, an LHP with a flat evaporator and two primary biporous wicks is studied for reducing the impact of heat leak on the compensation chamber. Liquid supply for the wick in the evaporator back is guaranteed by ensuring that the system is at a favorable elevation with slopes of 10° and 90° in the experiment. When the evaporator wall temperature is lower than 90°C, the maximum operational heat load can reach 270W with a slope of 90°, which corresponds to a heat flux of 26.5W/cm2. The maximum heat load is 210W with the 10° slope. The system can start up steadily with a low heat load of 10W, while a similar structure with single wick fails at this heat load. When heat loads are applied to both the evaporator wall and back simultaneously, the system has better operating performance for lower temperatures and thermal resistance. The wick in the evaporator back improves the performance at low heat load for the operation with only one side heat load, and plays a role in all operations with bifacial heat loads. As the heat load increases, the evaporating heat transfer coefficient increases to a maximum, and then settles maintaining a high value. The thermal resistance of the LHP with two primary wicks is lower for a larger slope. The minimum thermal resistance of the LHP is 0.218°C/W at 270W with a slope of 90°.

Influence of heating load on heat transfer characteristics in micro-pin-fin arrays

Experimental investigations were carried out to explore the convective heat transfer in micro pin-fins with different aspect ratios, and the influence of heating load on Nusselt numbers in micro pin-fins with liquid water as working fluid were investigated. The mechanism of convective heat transfer in micro pin-fins at different heating load were studied by 3-D numerical investigations, and the relationships of thermal physical properties change, the end wall effect and axial thermal conduction with Nu numbers in micro pin-fins were analysed. It was found that the thickness of boundary layer was decreased as much as 33.3 % attributed to the destructive effect of thermal physical properties change, and convective heat transfer in the micro pin-fin channel was more than 20 % enhanced by the flow disturbance caused by the increase of temperature difference. The discrepancy of Nu in micro pin-fin channel with different aspect ratios reached 34.59 %, and this discrepancy was reduced by the increase of heating load. The maximum value of impact factors of dynamic viscosity and thermal conductivity on the Nu in micro-pin-fins reached 25.02 and 7.68 %, respectively.

Enhancement of heat transport in oscillating heat pipe with ternary fluid

The thermo-physical properties of working fluids play an important role in the heat transport performance of an oscillating heat pipe (OHP). In the present study, the heat transport performance of the OHP is investigated using two types of binary fluids and a type of ternary fluid as the working fluids. The working fluids include a nanofluid, self-rewetting fluid, and a mixed solution of self-rewetting fluid and a nanofluid called self-rewetting nanofluid. The tilt angle of the OHP is 90° with a charge ratio of 50%. In contrast to de-ionized water used as a base working fluid in the OHP, all working fluids can enhance the heat transport performance of the OHP. However, through an analysis of the enhancement ratios, it is found that nanofluids can only enhance the performance of an OHP within a heat load range of 30–70W. The maximal enhancement ratio is about 11% as the heat load is 60W. If the heat load exceeds 70W, the heat transport performance of the OHP degrades. For a self-rewetting fluid, the maximum of enhancement ratio is only 6% as the heat load is less than 30W, and then, as the heat load increases, the enhancement ratio decreases gradually. The OHP that uses self-rewetting nanofluids shows excellent heat transport performance over the entire heat load range. The maximal enhancement ratio is approximately 15%. Therefore, self-rewetting nanofluids are considered to be appropriate working fluids for use in the OHPs.

Impact of a narrow limiter SOL heat flux channel on the ITER first wall panel shaping

The inboard limiters for ITER were initially designed on the assumption that the parallel heat flux density in the scrape-off layer (SOL) could be approximated by a single exponential with decay length λq. This assumption was found not to be adequate in 2012, when infra-red (IR) thermography measurements on the inner column during JET limiter discharges clearly revealed the presence of a narrow heat flux channel adjacent to the last closed flux surface. This near-SOL decay occurs with λq ∼ few mm, much shorter than the main SOL λq, and can raise the heat flux at the limiter apex a factor up to ∼4 above the value expected from a single, broader exponential. The original logarithmically shaped ITER inner wall first wall panels (FWPs) would be unsuited to handling the power loads produced by such a narrow feature. A multi-machine study involving the C-Mod, COMPASS, DIII-D and TCV tokamaks, employing inner wall IR measurements and/or inner wall reciprocating probes, was initiated to investigate the narrow limiter SOL heat flux channel. This paper describes the new results which have provided an experimental database for the narrow feature and presents an ITER inner wall FWP toroidal shape optimized for a double-exponential profile with λq = 4 (narrow feature) and 50 mm (main-SOL), the latter also derived from a separate multi-machine database constituted recently within the International Tokamak Physics Activity. It is shown that the new shape allows the power handling capability of the original shape design to be completely recovered for a wide variety of limiter start-up equilibria in the presence of a narrow feature, even taking assembly tolerances into account. It is, moreover, further shown that the new shape has the interesting property of both mitigating the impact of the narrow feature and resulting in only a very modest increase in heat load, compared to the current design, if the narrow feature is not eventually found on ITER.

Modeling and Dynamic Simulation of a Large Scale Helium Refrigerator

In order to simulate the transient behaviors of a newly developed 2kW helium refrigerator, a numerical model of the critical equipment including a screw compressor with variable-frequency drive, plate-fin heat exchangers, a turbine expander, and pneumatic valves wasdeveloped. In the simulation,the calculation of the helium thermodynamic properties arebased on 32-parameter modified Benedict-Webb-Rubin (MBWR) state equation.The start-up process of the warm compressor station with gas management subsystem, and the cool-down process of cold box in an actual operation, were dynamically simulated. The developed model was verified by comparing the simulated results with the experimental data.Besides, system responses of increasing heat load were simulated. This model can also be used to design and optimize other large scale helium refrigerators.

The combined effects of magnetic asymmetry, assembly and manufacturing tolerances on the plasma heat load to the ITER first wall

The base plasma heat load on the ITER first wall results from the projection of the plasma scrape off layer power profile onto the shaped surface of the first wall panels. The actual heat load on the first wall is larger than the base heat load, due to enhancement factors such as design approximations, manufacturing and assembly errors, and deviations to the ideally toroidal magnetic field configurations. The heat load increase is accounted for through penalty coefficients, ranging individually from a few percent to 50%. The probabilistic combination of individual penalty coefficients to give a global penalty coefficient is presented in the paper. The global penalty coefficient ranges from 1.37 to 2.44, depending of the first wall row and plasma phase.

Impact of pressure on high intensity colorless distributed combustion

In recent years, colorless distributed combustion (CDC) has been shown to provide ultra-low pollutants emission, enhanced stability, fuel flexibility and thermal field uniformity. To achieve CDC conditions, fuel–air mixture must be properly prepared prior to the mixture ignition. In this paper, the impact of moderate pressure increase on a test combustor is examined with emphasis on pollutants emission under near CDC conditions. Increase in pressure at constant mass flow rate resulted in a significant increase in NO emission and departure from CDC conditions. This is a result of reduction of the volume flow rate and injection velocity with pressure increase, as well as an increase in the apparent residence time of gases within the combustor. These conditions led to less than adequate mixing and more residence time of gases in the post flame zone to form NOx. CO emissions were reduced as longer residence time allowed for complete conversion of CO to CO2. Also, the increase in pressure enhanced chemical kinetics and suppressed dissociation. Increasing the combustor pressure with constant heat release intensity, where mass flow rates are increased to keep the apparent residence time constant, showed a slight increase (1PPM) of NO under premixed conditions and significant reduction of CO. OH∗ chemiluminescence showed that the reaction zone did not change with increase in pressure under premixed combustion. For non-premixed combustion, the reaction zone intensity increased and was more concentrated with increase in pressure to result in higher emissions as compared to the premixed case. In both cases, CO emission was significantly reduced due to the increased heat load and temperatures within the combustor. Relations describing change of NO and CO emissions with pressure were used to predict the combustor emissions at higher pressures. Difference between premixed and non-premixed modes revealed the need for more adequate mixing, necessary for CDC condition. Increase in pressure without considering flow velocities and flowfield can lead to departure from CDC conditions with simultaneous increase in emissions.

Investigation of the displacement effect of a diffuse ceiling ventilation system

This paper reports on an experimental and analytical investigation on the potential displacement effect of a diffuse ceiling ventilation system. Experimental data was generated in a test chamber where measurements of four test series were conducted for two different placements of the exhaust. Furthermore, CFD calculations were performed to support the analysis of the experimental results. The results show that tendencies known from conventional displacement ventilation occur at low heat loads and a development towards full mixing with increasing heat load. It has been demonstrated that the vertical air velocity profiles, temperature distribution and ventilation effectiveness are independent of the Archimedes number.