Increasing Heat Load Research Articles

Visualized experimental study on steady-state performance of a loop heat pipe under elevated acceleration fields

In this article, an experimental investigation was carried out to explore the steady-state operational behavior of a newly designed nickel-water dual compensation chamber loop heat pipe (DCCLHP) under elevated acceleration fields. Glass windows were fabricated and embedded on a condenser and two compensation chambers (CCs) for the purpose of visual observation of vapor and liquid distribution. The steady-state operational behaviors and any new phenomena were discussed. The heat load from 30 W to 110 W and the acceleration magnitude from 1 g to 13 g as well as four different acceleration directions were taken into account. Experimental results indicate that: (i) as the heat load increases, the steady-state operational temperature of the DCCLHP increases, while thermal resistance may increase or decrease, also depending on both the acceleration direction and magnitude. (ii) The thermal resistance at both direction A and D is significantly larger than that at both direction B and C. The minimum thermal resistance with 0.78 K/W at 11 g with 70 W and operational temperature with 72.7 °C at 3 g with 30 W are achieved at direction C. (iii) as the acceleration magnitude increases, both operational temperature and thermal resistance tend to decrease for both direction B and C, whereas an opposite tendency is observed for both direction A and D. (iv) both the vapor column and liquid column alternatively distribute in the condenser channel that could increase or decrease the loop pressure drop. Inclined vapor–liquid interface and different liquid amount in both CCs are formed at different accelerations. Stratified flow or annular flow could lead to the film condensation heat transfer in the condensation channel.

Flat-Plate PHP with Gravity-Independent Performance and High Maximum Thermal Load

In many energy-related applications, components with high heat loads, such as power electronics, play an important role. Pulsating heat pipes (PHPs) are an effective solution to deal with the increasing heat load of these components. In many real-life applications, the PHP must work against gravity and still be able to operate efficiently. However, the majority of present flat-plate PHP designs do not perform well under this condition. Therefore, this paper presents a flat-plate PHP with a conventional channel design optimized for gravity-independent operation. The PHP was capable of transmitting a heat output of 754 watts in all orientations, while the testing heater in use never exceeded a temperature of 100 °C. No indications of dryout were observed, implying that the maximum thermal load the PHP can handle is even higher. Additionally, three different condenser zone sizes were tested with the PHP. Previously published results indicated that there is a specific range of suitable condenser zone sizes, and performance problems will occur if the condenser zone size falls outside of this range. The findings from this work point in the same direction.

Impact of indoor heat load and natural ventilation on thermal comfort of radiant cooling system: An experimental study

Construction and operation of buildings are responsible for about 20% of the global energy consumption. The embodied energy of conventional buildings is high due to the utilization of energy-intensive construction materials and traditional construction methodology. Higher operational energy is attributed to the usage of power-consuming conventional air-conditioning systems. Therefore, moving to an energy-efficient cooling technology and eco-friendly building material can lead to significant energy savings and CO2 emission reduction. In the present study, an energy-efficient thermally activated building system (TABS) is integrated with glass fiber reinforced gypsum (GFRG), an eco-friendly building material. The proposed hybrid system is termed the thermally activated glass fiber reinforced gypsum (TAGFRG) system. This system is not only energy-efficient and eco-friendly but also provides better thermal comfort. An experimental room with a TAGFRG roof is constructed on the premises of the Indian Institute of Technology Madras (IITM), Chennai, located in a tropical wet and dry climate zone. The influence of indoor sensible heat load and the impact of natural ventilation on the thermal comfort of the TAGFRG system are investigated. An increase in internal heat load from 400 to 700 W deteriorates the thermal comfort of the indoor space. This is evident from the increases in operative temperatures from 29.8 to 31.5 °C and the predicted percentage of dissatisfaction from 44.5% to 80.9%. Natural ventilation increases the diurnal fluctuation of indoor air temperature by 1.6 and 1.9 °C for with and without cooling cases, respectively. It reduces the maximum indoor CO2 concentration from 912 to 393 ppm.

EMERGENCY COOLING OF SUPERHEATED SURFACES BY NANOFLUIDS ADDITIVES IN STOP- AND NON-STOP MODES OF HEAT LOAD RISE

Early [Technical Physics Letters, 2016, Vol. 42, P. 677–681. — https://doi.org/10.1134/ S106378501607004X] we have shown the possibility of emergency cooling of an overheated Ni/Cr surface using additives of aluminosilicate nanofluids (AlSi-NF) at the time of developed film boiling of water (crisis), provided that the increase in thermal load is stopped and its fixation at the level of Q » 1.0 MW/m2, exceeding the critical heat flux (CHF) of water (0.7 MW/m2). However, in real operating conditions of cooling systems (especially for nuclear reactors), emergency situations sometimes arise in which it is very problematic to immediately turn off the heat load supply or maintain it at a certain predetermined level. In this regard, in this work, on a fully automated stand, the peculiarities of eliminating the water boiling crisis and emergency cooling of the overheated surface of the mini-reactor heater by injecting a portion of hot AlSi-NF in conditions of film boiling of water and a steady increase in heat load were studied, and the results were compared with previously obtained in the mode of stopping the rise of the thermal load. The test was carried out on an aqueous AlSi-NF nanofluid obtained on the basis of a natural mixture of aluminosilicates montmorillonite + palygorskite (Ukraine). The boiling-overheating-cooling curves, as well as the time dependences of the heat transfer coefficient and the heating surface temperature, were recorded in automatic real-time mode. Emergency cooling of the overheated surface (from 600 to 125 °C) after the introduction of a portion of hot AlSi-NF occurred in a matter of minutes due to a sharp increase in the heat transfer coefficient α up to 55,000 W/(m2.K). Such a phenomenon of a sharp intensification of heat transfer and a 3-fold increase in the specific heat flux (qsp) during boiling of AlSi-NF compared to the base liquid (water) is explained by the deposition on the heating surface of a gel-like layer of nanoparticles with high hydrophilicity and mobility, which can sharply increase nucleate boiling and convection. Regardless of the mode of supplying the heat load, the principal possibility of overcoming the boiling crisis and emergency cooling of the superheated surface with the addition of AlSi-NF nanofluid has been established, for a time sufficient to eliminate the accident. Bibl. 27, Fig. 4, Tab. 1.

Relationship between phase distribution and heat-transfer coefficient of loop heat pipe evaporator investigated by lateral observation of porous media

Loop heat pipes (LHPs) can be passively operated, require no electric power, and can be flexibly arranged, making them promising in thermal control systems for both terrestrial and aerospace applications. Understanding the two-phase state in the porous media of a capillary evaporator induced by nucleate boiling is key to enhancing the evaporator performance. This study visualized the liquid–vapor (L–V) phase distribution on the contact surface between the cylindrical evaporator casing and the wick to investigate the L–V phase distribution and to figure out its relationship with the evaporator heat-transfer coefficient. The visualization was successfully realized under conditions similar to those observed in a practical environment using a sapphire tube as the evaporator casing. The observation results showed that the wick underwent a phase transition from a liquid-saturated state to a two-phase state under the effect of nucleate boiling, and the evolution of the L–V phase distribution on the contact surface with varying heat load could be determined. The heat-transfer coefficient peaked immediately after the nucleate boiling and then decreased with increasing heat load. The three-phase contact line (TPCL) within the casing, liquid, and vapor phases showed a similar trend; however, the saturation on the contact surface was different, decreasing quickly after the nucleate boiling, followed by a gradual decrease. The correlation coefficients between the heat-transfer coefficient and the saturation/TPCL length were −0.61 and 0.83 under all heat load conditions, respectively. The characteristics of the heat-transfer coefficient as a function of the heat load were found to correspond to those of the TPCL length rather than the saturation. This finding can aid in the design of microscopically optimized porous media by additive manufacturing.

Design and Thermal Performance of Ultra-Thin Flat Plate Heat Pipes with Composite Wick for Electronic Cooling

The rapidly increasing heat load and high-density packaging of electronic devices demand heat dissipation devices that possess miniaturization and high thermal performance. A series of ultra-thin flat plate heat pipes (UPHPs) with thicknesses of 1.2 mm was prepared to satisfy this demand. Strengthening rib optimization and capillary optimization were processed to restrict the entrainment limit and capillary limit of the UPHPs. Furthermore, surface modification technology with composite nanostructure coated was introduced to improve the surface wettability of capillary wick. Finally, the heat transfer capacity of the UPHPs was experimentally tested. The influence of capillary structure and liquid filling ratio on the heat transfer characteristic was investigated in detail. This work provides insight into the design and application of UPHPs for the new generation of electronic devices.

Building energy demand of urban blocks in Xi'an, China: Impacts of high-rises and vertical meteorological pattern

High-rise buildings have been promoted in high-density cities. The impact degree of urban high-rises on the building energy demand at the urban block scale is determined by many factors, including the effects of the vertical meteorological pattern (VMP), which have been discussed insufficiently. In this study, the VMP of urban areas in Xi'an, China was observed, and a building energy simulation model that revealed the “urban block form–outdoor climate–building energy” nexus was developed with the URBANopt plugin on the Grasshopper platform. The impacts of the VMP on the building heating and cooling energy demands of different urban blocks were then predicted. In January, the VMP led to building heating load increases of 0.7–1.6 kWh/m2 (16–52%) and 0.0–1.1 kWh/m2 (0–35%) in the residential and mixed-use blocks, respectively. In July, the ranges of the building cooling load changes produced by the effects of the VMP were 0.0–0.1 kWh/m2 (0–1%) and −0.3–0.2 kWh/m2 (−2%) in the residential and mixed-use blocks, respectively. Furthermore, the VMP affected the relationship between the urban block form and the building energy demand. Based on the relationship, we provided form-optimization advice for different functional urban blocks. The urban high-rise mechanisms in high-density cities and their impacts on the building energy demand were discovered in this study. This can help decision-makers to optimize urban planning while promoting urban sustainability.

Effect of key structure and working condition parameters on a compact flat-evaporator loop heat pipe for chip cooling of data centers

Loop heat pipe has the advantages of large heat transfer coefficient without extra power input, therefore it has a good application potential in chip cooling of data centers. In this paper, a compact flat-evaporator loop heat pipe is designed and the effects of key structural parameters and working conditions on the dynamic and steady-state performance are studied. The results show that increasing heat load or the filling ratio, arranging vapor channels in the middle of the wick (rather than at the bottom) or lengthening the condenser can all reduce the temperature fluctuation; A small inclination angle of 5° and 10° can reduce the temperature fluctuation and the stable temperature, while large inclination angle of 15° will instead bring higher stable temperature; When the heat load increases step by step, the stable temperature (average temperature for fluctuation conditions) decreases and then increases for all filling ratios and structures; Minimized thermal resistance is achieved by optimizing wick and condenser structure under low and high heat load, respectively.

ИССЛЕДОВАНИЕ ОСОБЕННОСТЕЙ ИСПОЛЬЗОВАНИЯ НОРМАЛИЗОВАННОЙ ТЕПЛОВОЙ НАГРУЗКИ ДЛЯ ОПРЕДЕЛЕНИЯ УРОВНЯ ЭНЕРГОЭФФЕТКИВНОСТИ КОТЕЛЬНОГО АГРЕГАТА В КОНКРЕТНЫХ УСЛОВИЯХ

When comparing the efficiency gross of the same units operating under different conditions, or the same unit for different periods of time, often does not take into account that these values are obtained under different conditions. It is well known that the efficiency of the boiler unit depends on the heat load of the boiler. As a rule, the value of efficiency maximum at moderate loads of the boiler unit and then with increasing heat load gradually decreases according to a law close to linear. Thus, it is possible that for a unit with a better technical condition, an efficiency value will be obtained that is lower than that of another unit operating in more favorable conditions. There is also ambiguity in the presentation of the efficiency dependence. boiler unit from heat load. The paper studies the issue of the features of using the normalized heat load to determine the level of energy efficiency of a boiler unit based on the TVG-4r boiler of a small quarterly boiler house. It is shown that a feature of using the reduced efficiency, which is determined depending on the normalized heat load, is that this indicator characterizes not only the actual technical condition of the boiler unit, but also the efficiency of its regulation, taking into account changes outside air temperature

Shade of solar panels relieves heat load of sheep

Use of solar panels as shading resource combines better animal welfare, generation of renewable energy and animal production in the same area. This concept is termed Animal Agrivoltaics. In this study we integrated behavioural and physiological data in order to determine benefits provided by shade of solar panels on heat stress indicators of sheep. Nine adult Corriedale ewes (Body mass: 70 ± 12 kg; fleece length: 70 ± 5 mm) had miniature temperature loggers attached on them for every-minute samplings of wool-surface, skin and vaginal temperature, from 08:00 to 17:00 h, along 20 consecutive days. Ewes also had shade-use behaviour directly and continuously monitored. Ewes were kept in a paddock shaded with a roof structure that consists of ten photovoltaic panels (shade availability: 2.14 m2 animal−1). Over the study period, ewes were exposed to levels of solar radiation as much as 900 W m−2. From 08:00 to 17:00 h, regardless of the duration of sun exposure or in the shade of solar panels, the vaginal temperature of ewes remained within a narrow range (38.5 ºC - 39.2 ºC). At levels of solar irradiance above 500 W m−2, ewes increased wool-surface temperature by 10 ºC and skin temperature by 2 ºC with increasing heat load and time exposed to sun. Under such circumstances, they were more likely to move to shade of solar panels. Between 10:00 and 14:00 h, shade of solar panels provided to ewes 30 % less radiant heat load than if they were exposed to sun. As a result, as the time in shade increased, the wool-surface temperature of ewes was reduced by 7 ºC - 8 ºC, and skin temperature by 1.0 ºC - 1.5 ºC. In conclusion, this study provides strong evidence that shade of solar panels efficiently reduces heat load in sheep.

Experimental study on the thermo-hydrodynamic characteristics of a nitrogen pulsating heat pipe

A simultaneous thermal and visual experiment is conducted on a silicon-based flat-plate nitrogen pulsating heat pipe (PHP) for the first time. The PHP operates successfully with six filling ratios (FRs) between 25% ∼ 76% under bottom heating mode. Start-up and quasi-steady state performances are analyzed. Two types of start-up processes induced by bubble nucleation and interface evaporation are observed, and different start-up mode results in different degrees of thermal performance hysteresis phenomenon. The optimal thermal resistance with different FRs is always lower in a lower FR working condition, but a lower FR results in a lower heat transfer limit. The optimal FR rises with increasing heat load range. The fluid flow data are quantified in two aspects, which are flow intensity and velocity, and coupled with the heat transfer data for analysis. It is inferred that the latent heat transfer efficiency in the nitrogen PHP is higher than that of the sensible heat transfer.

Selection of turbulence promoters for retrofit applications through thermohydraulic performance mapping

Many new heat transfer enhancement devices have been reported in the open literature in the past decades exhibiting innovative geometries and better thermal and hydraulic performance. For a practical application in tubular heat exchangers, the selection of the best inserts out of the many different geometries available can be a cumbersome task. This work shows an approach to reduce the complexity of the search by graphically displaying the relative increase of heat load and pressure drop by means of performance mapping plots. The analysis includes the use of Nusselt and friction correlations and their application at different Reynolds numbers and different fluid viscosities. Out of the different geometries available, a group of 51 are compared in terms of the effect on heat duty and pressure drop. The approach is extended to analyse the exchanger irreversibility. For different operating conditions and fluid types, it is shown that the perforated twisted tape is the best performing insert. This insert produces an increase in heat transfer rate between 4% and 7.3% and an increment in pressure drop between 2 and 3.43 times that of the base case.

Numerical study identifies the interaction between two adjacent dry cooling towers on fluid flow and heat transfer performances of the radiators at different points of each tower

A reliable thermal power plant in arid area usually has twin power-generating units equipped with two dry cooling towers. For a twin-tower configuration, a three-dimensional numerical model was built to capture thermo-hydraulic characteristics of both towers and distribution patterns of heat transfer performances of their radiators. Results showed that the interaction between two adjacent towers is a combination of negative blocking effect and positive wind-breaking effect of one tower to another, and its influences on radiators of different orientations are distinct and dependent on wind direction and speed. As wind attack angle increases, the overall cooling performance of the upstream tower improves and its sensibility to wind speed declines, while performance variation of the downstream tower shows a decrease followed by an increase. The downstream tower performs worse than the upstream tower in most crosswind cases, except for the wind situation where the downstream tower is almost entirely blocked by the upstream one. For a given wind speed of 12 m/s, cooling efficiency of the upstream (downstream) tower rises from 69.4% to 73.7% (91.7%) as wind attack angle varies from 0° to 90°, causing condenser pressure of the associated power unit to reduce from 23.5 kPa to 20.5 kPa (14.4 kPa). As the tower spacing increases, the tower interaction mitigates and cooling performance difference between the two towers narrows. The decrease of ambient temperature barely impacts cooling efficiency of the tower, but does cause a drop in condenser pressure of the unit without changing the sensitivity of condenser pressure to crosswind. The increase in condenser heat load helps to improve cooling tower performance and reduces its sensitivity to crosswind. Whereas, the growth in condenser heat load dominates the increases in condenser pressure and its sensitivity to crosswind.

Experimental investigation of the cooling capacity and thermal performance of the passive displacement dual cooling coil system in tropical climate

This paper focuses on the application of Passive Displacement Dual Cooling Coil (PDDCC) system in an indoor space based on a test lab. Parametric study, first, was carried out using Computational Fluid Dynamics (CFD) simulation to identify the optimum fall duct design for the PDDCC unit. Thermal profiles obtained through the simulations indicated that the fall duct with 300 mm height and 0° taper angle was most suitable as it generated the largest volume of cool air which sufficiently cooled the space. It was then applied to the experiment of the PDDCC units in the test lab to study the system's thermal performance. The total cooling load removed by the two PDDCC units within the indoor space varied from 2.24 kW to 2.93 kW. The latent load removal over the entire experiment duration ranged from 0.7 kW to 1.35 kW where the highest latent heat was removed at the lowest chilled water setpoint temperature of 7.5 °C. Thermal stratification trends displayed that temperature within the room for all chilled water setpoint temperatures (7.5 °C, 8.5 °C, 9.5 °C, and 10.5 °C) corresponded with the increase in heat load within the indoor environment. The humidity level above 1 m ground level was not exceed 56%. The PDDCC system was found to have excellent latent heat removal capability. The PDDCC system was able to produce a thermally comfortable environment for 67% of the experimental permutations. The low humidity level within the test cell allowed to raise the boundary of the acceptable temperature range for achieving human thermal comfort to as high as 26.5 °C.

Numerical and experimental investigation of a novel liquid upper-feeding micro-channel flat loop thermosyphon cooling and heat recovery system

ABSTRACT A novel liquid upper-feeding micro-channel flat loop thermosyphon (LUF-MCFLTS) system was developed, to eliminate the drying phenomenon of the upper surface in the traditional LTS and provide a highly efficient solution for DC cooling and heat recovery. The effects of cooling capacity (cooling water flow rate and temperature), heat load, and filling ratio on the heat transfer performance of the LUF-MCFLTS system are investigated experimentally. The results indicated that with the increase in cooling water flow, the decrease in cooling water temperature, and the increase in heat load, the heat transfer performance of the LUF-MCFLTS system was better. Under a heat load of 500 W, a cooling water flow rate of 600 L/h, and a cooling water temperature of 15°C, the heat recovery efficiency could reach 86.28%. Compared with the MCFTS and MCFLTS systems in literature, the recovery efficiency of the LUF-MCFLTS system could be increased by 31.6% and 2.64%, respectively. In addition, an effective numerical model to predict the heat recovery efficiency of the LUF-MCFLTS system with the experimental data in the 2.07% error band has been established. This work provides a new idea and theoretical basis for the design of DC cooling and heat recovery system.

Non-premixed combustion and NOX emission characteristics in a micro gas turbine swirl combustor fueled by methane and ammonia at various heat loads

In comparison to methane (CH4), ammonia (NH3) is considered a potential carbon-free alternative fuel that can reduce greenhouse gas emissions. But a principal concern is the generation of elevated nitrogen oxide (NOX) emissions from NH3 flame. In this study, the detailed reaction mechanisms and thermodynamic data of CH4 oxidation and NH3 oxidation were performed using the steady and unsteady flamelet models. After validation of the turbulence model, the combustion and NOX emission characteristics of CH4/air and NH3/air non-premixed flames in a micro gas turbine swirl combustor under a series of identical heat loads were numerically investigated and compared. The present results show that the high-temperature zone of the NH3/air flame migrates more rapidly toward the outlet of the combustion chamber than that of the CH4/air flame as the heat load increases. The average NO, N2O, and NO2 emission concentrations at all heat loads from NH3/air flame are respectively 6.12, 161.05 (given the very low N2O emission concentration from CH4/air flame), and 2.89 times higher than those from CH4/air flame. There are correlation trends between some parameters (e.g. characteristic temperature and OH emissions) with the variation of the heat load, and the relevant parameters can be tracked to predict the emission trends after changing the heat load.

Short-term physiological responses to moderate heat stress in grazing dairy cows in temperate climate.

Even in temperate climate regions, an increase in ambient temperature and exposure to solar radiation can cause heat stress in lactating dairy cows. We hypothesised that grazing dairy cows exhibit short-term physiological changes due to increasing heat load under moderate climate conditions. Over two consecutive summers, 38 lactating Holstein dairy cows were studied in a full-time grazing system. Data were collected in 10 experimental periods of up to three consecutive days with a moderate comprehensive climate index (CCI). The individual animals' vaginal temperature (VT), heart rate, and locomotor activity data were automatically monitored with sensors. Blood samples and proportional whole milk samples were collected at afternoon milking. The concentrations of beta-hydroxybutyrate, glucose, non-esterified fatty acids, urea nitrogen, plasma thyroxine and triiodothyronine were analysed in blood plasma, and fat, protein, lactose, urea nitrogen, cortisol, Na+, K+, and Cl- concentrations were analysed in milk. The daily distribution of VT recordings greater than 39°C showed a circadian rhythm with a proportion of recordings of 2% and lower during the night and a percentage of 10% or higher in the afternoon. The cows' maximal daily vaginal temperature (VTMAX) between 0830 and 1430h was positively related to the mean daily CCI in the same time period (CCIMEAN; mean and SD 23.6±5.4°C). Cows with greater VTMAX had an increased mean heart rate, plasma glucose and milk cortisol concentrations and decreased concentrations of plasma thyroxine and triiodothyronine. The concentration of Na+ in milk was lower, and the concentration of K+ in milk tended to be higher in cows with increased VTMAX. For beta-hydroxybutyrate, non-esterified fatty acids and urea nitrogen concentrations in plasma and fat and lactose concentrations in milk no relationships were found in terms of increasing VT. For milk urea nitrogen and protein concentrations, the proportion of total variance explained by inter-individual or -period variance was high. In conclusion, changes observed in milk and blood likely reflected short-term physiological responses to moderate heat stress. In particular, milk cortisol and Na+ may be useful traits for timely monitoring of heat stress in individual cows because their inter-individual variances were relatively small and samples can be collected non-invasively.

Evaporative cooling via panting and its metabolic and water balance costs for lizards in the American Southwest.

In lizards there is considerable variation in the ability to dissipate environmental/endogenous heat loads through evaporative cooling via panting, which effects how long lizards can spend exposed to high solar heat loads. We recently described the differing capacities of lizards to depress body temperature (Tb) through evaporative cooling via panting. Here, we link panting and Tb depression with rates of evaporative water loss and its metabolic costs under high heat loads. We used flow-through respirometry to measure evaporative water loss rates and metabolism of 17 lizard species from the American Southwest while simultaneously measuring Tb. We exposed lizards to air temperatures (Ta) ranging from 35°C to their critical thermal maximum (CTmax) while marking the onset of panting. We then estimated pre-panting Q10 values for metabolism to partition increases in metabolism associated with the van't Hoff effect from the mechanical cost of panting with increasing heat loads. We found that evaporative cooling costs substantially varied among species, with panting effort significantly affecting lizards' evaporative capacity. Lizard evaporation rates ranged from 0.32 to 1.5 g H2Oh-1, with individuals losing as much as 6%h-1 of body mass while panting. Lizards also experienced an increase of up to 7.9-fold in metabolic rate while panting, although the overall energetic costs of panting remained relatively low compared with evaporative water costs. Across species, there was a significant positive relationship between the overall rate of evaporative heat loss and the maximum Ta-Tb gradient a species could maintain. While evaporative cooling may be an effective mechanism for reducing Tb and extending activity in hot environments for many species, it has significant metabolic and water balance costs that should be considered, as habitats with high environmental heat loads can be especially costly to an animal's water budgets.

Experimental study on the passive regulation of a CO2 two-phase thermosyphon loop

• The passive regulation mechanism of CO 2 TPTL was revealed. • Three typical operation states of the TPTL occurred as the heat load increased. • Each operation state showed different characteristics as the filling ratio changed. • The largest normal operation heat load range was found under 45% filling ratio. Vapor–liquid distribution and flow characteristics in a two-phase thermosyphon loop usually exhibit regular variation under varying working conditions, which can be referred to as passive regulation. Currently, the passive regulation law of a CO 2 two-phase thermosyphon loop has not been well understood. In this study, experiments were performed to examine the passive regulation mechanism of a CO 2 two-phase thermosyphon loop under different heat loads and filling ratios. Local flow visualization was adopted, combined with the measurement of operating parameters. The CO 2 two-phase thermosyphon loop entered the pre-start, oscillatory, and stable operation stages successively with the increase in heat load. Vapor–liquid counterflow was observed in the downcomer in the pre-start stage. Temperature change due to thermal unbalance between the vapor and liquid can be observed along the riser or the downcomer. In the oscillatory stage, different types of oscillation were observed under different filling ratios. In the stable stage, the two-phase thermosyphon loop worked under good conditions until the heat transfer limit was reached. The two-phase thermosyphon loop reached its maximum heat transfer limit of 1200 W at a filling ratio of 45%. The normal operation heat load range of the two-phase thermosyphon loop was 380–650 W, 320–770 W, 700–1100 W, 600–1200 W, and 400–630 W for filling ratios of 20%, 25%, 40%, 45%, and 50%, respectively.

Voluntary heat stress abatement system for dairy cows: Does it mitigate the effects of heat stress on physiology and behavior?

Many cooling strategies are used to keep cows in thermal homeostasis; however, most of them are applied to the group level, commonly at the feed bunk or milking parlor. The variance of heat stress effects on animals are well known, but with more individualized management in dairy farms, group cooling opportunities are becoming restricted. It is known that dairy cattle are variable in their responses to an increase in heat load. Thus, the first objective of this study was to investigate the effect of 2 mandatory soakings at the exit of the milking parlor and free access to a voluntary soaking system compared with cows with access to a voluntary soaking system only, with no mandatory soakings. The second objective of this study was to assess the heat abatement capability of voluntary soaking of cows by assessing cow physiology, behavior, and milk production. Last, this study aimed to determine the individual use of the voluntary heat abatement system and its relationship with temperature-humidity index (THI). Fifteen mid-lactation Holstein cows were enrolled in this study and had free access to a motion-activated soaker (Cool Sense, Edstrom) located adjacent to the research pen for an 8-wk data collection period. Cows were paired according to parity, milk production, and body weight, and assigned a treatment with or without mandatory soakings twice per day. In the mandatory soaking treatment (MS), cows were soaked using a motion-activated soaker at the exit of the milking parlor and had free access to the voluntary soaker in the pen. Cows in the treatment without mandatory soakings (NMS) were not soaked at the exit of the milking parlor and had free access to the voluntary soaker in the pen. The effects of soaker treatment were analyzed using mixed linear models. The model included treatment, soaker uses per day, pair, mean daily THI, days in milk, daily milk yield, and interaction of treatment with mean daily THI. Study day was specified as a repeated measure, and cow as the subject, using an autoregressive structure. Also, we assessed the relationship of mean soaker use and THI against all variables. There was great individual variation in voluntary soaker use, ranging from 0 to 227 soakings/d (mean ± standard deviation, 13 ± 30 voluntary soakings/d). Treatment did not affect voluntary soaker use (MS, 12.4 ± 1.4 soakings/d; NMS, 14.8 ± 1.4 soakings/d), respiration rate (MS, 57.3 ± 0.4 breaths/min; NMS, 56.4 ± 0.4 breaths/min), or milk yield (MS, 36.5 ± 0.6 kg/d; NMS, 36.2 ± 0.6 kg/d). However, MS cows spent more time ruminating (MS, 558.6 ± 5.2 min/d; NMS, 543.4 ± 5.4 min/d). Temperature-humidity index had a positive relationship with voluntary soaker use and mean respiration rate. In conclusion, voluntary soaker use related positively to the THI, but no major productive, physiological, or behavioral differences were observed between soaking treatments. Furthermore, we found that voluntary soaker use is highly variable among cows and it was related positively to milk yield, where higher producing cows used the soaker more frequently.