Thermal Crisis Research Articles

Impact of variable properties in natural convection airflows within simple and adiabatically-extended vertical tubes

Influence of variable thermophysical properties is investigated in airflows established by natural convection inside vertical tubes. Systematic numerical results are obtained for covering the entire range laminar to turbulent (modified Rayleigh number ranging 10−3–109), for both isothermal and isoflux heating conditions. Particular attention is devoted to the effects of variable properties of air. Own functions are used to take into account the variation of thermophysical properties as a function of temperature. Results are compared to those computed assuming the Boussinesq approach. For isoflux heating, it is proven that the disruptive effect consisting of the emerging of a critical heat flux and the consequent thermal crisis phenomenon, appear in the regarded cylindrical geometry. An analytical estimation of the critical heat flux is developed for vertical tubes, valid for the laminar regime, and an overall correlation is proposed for the laminar-turbulent range. The impact of the height and the widening of a funnel-shaped adiabatic extension is explored for selected values of the modified Rayleigh number, and the conditions under which its inclusion is beneficial to postpone the appearance of the crisis phenomenon, are determined.

Enhanced electrolytic immersion cooling for thermal crisis mitigation in high-energy–density systems

Motivated by the increasing need for effective cooling solutions in high-energy–density systems, this experimental study presents the two-phase cooling of a superheated SS 316L sample via immersion quenching in saturated deionized (DI) water at atmospheric pressure conditions with and without a DC electric field. We investigate the effect of the applied electric field, electrode polarity, and in-situ oxidation on quenching characteristics such as the cooling profile, vapour layer behaviour, minimum film boiling temperature (Tmin), and heat transfer rate. The cooling curves of samples quenched with the application of an electric field shift towards the left compared to the sample quenched without an electric field. The cathode sample at 200 V exhibits 33 % faster cooling than the bare sample (0 V). Overall, the in-situ oxidised SS 316L cathode sample at 200 V exhibits a 55 % reduction in film boiling duration, and Tmin increased from 268 °C to 322 °C compared to the bare sample (0 V). The visualisation studies highlight that the liquid–vapour interface experiences a series of oscillations followed by temporal collapse due to electrostatic attraction and electrolytic activity. The obtained results show that hydrogen-rich vapour bubbles increase heat transfer performance. The evolution of hydrogen and its adsorption at the sample surface reduces the activation energy for bubble nucleation and improves the bubble density via liquid pumping. These insights open the pathway for employing hydrogen bubbles for handling ultra-high thermal loads in high-energy density systems, and the specific case of a revised design of concentrating solar receiver is considered based on the present findings.

Innovative experimental approach for the dynamic Multi-Variable investigation of Pulsating heat Pipes

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the startup and the thermal crisis. An innovative experimental apparatus is designed to systematically investigate the above-mentioned phenomena. It consists in a square loop made of four borosilicate transparent glass tubes joined at corners by means of brass connectors. The external tube surface is coated with several transparent Indium Tin Oxide heaters. The device is used to topologically reproduce 5, 7, and 11 evaporator/condenser sections PHPs with a 2 mm inner diameter tube, filled with pure ethanol and tested in horizontal position. First a steady-state characterization is performed. The condenser temperature is varied from 10 °C to 40 °C; the input power goes from 10 W to 40 W. Results show that both the increase of number of evaporator sections and condenser temperature enhance the PHP performance in terms of equivalent thermal resistance. Then, the effect of the condenser temperature on the initial liquid phase distribution is analyzed. It is observed that at low condenser temperatures (10 °C, 20 °C), the liquid phase tends to gather in the condenser sections rather than the evaporator sections and it is arranged in short liquid plugs; conversely, for high condenser temperatures (30 °C, 40 °C), the amount of liquid phase in evaporators is higher and it is arranged in long liquid plugs. Finally, the transient behavior is studied in terms of startup time. It is observed that the parameter influencing most the startup time is the initial fluid distribution; the startup time increases with the increase of evaporator sections volume occupied by liquid phase and with the length of liquid plugs; on the other hand, the startup time decreases with the increase of number of menisci in the evaporator sections.

Research and Experience Reference on London’s Response to Climate Change in the Twenty-first Century

London’s approaches to tackling climate change after the 21st century are multifaceted and relatively systematic. The aim of this research paper is to analyse London’s actions in response to climate change and to draw out what valuable lessons London has for the world in terms of its response to climate change. This paper provides an in depth analysis of London’s policies and actions on climate mitigation in the areas of “greenhouse gas emissions” and “energy infrastructure”, and climate adaptation actions in the areas of “city green belt and urban afforestation”, “UHI and thermal crisis management” and “water supply infrastructure and sustainable drainage”. It then examines the positive aspects of these actions to determine what London has to say about climate change to the rest of the world and other cities. This paper also discovers that to effectively mitigate and adapt to climate change, London has not only established carbon reduction targets, but also created a large academic research network, represented by the LCCP. At the same time, London has developed a scientific climate change adaptation planning framework (P2R2) that focuses on four key areas: Economic, environmental, health, and infrastructure sectors, and three types of risks: Flooding, heat, and water supply, and emphasizes the dynamics and flexibility of each adaptation strategy

Can cyanotoxins explain the clinical features of the thermal crisis in balneotherapy?

Microbial biofilms communities in mineral waters and hot springs have a particular composition with species belonging to different groups such as epsilonproteobacteria and gammaproteobacteria or different siderobacteria and other chymoautrophic organisms, in addition to certain bacillaryophytes, chlorophytes and especially cyanobacteria. Balneotherapy can cause adverse reactions to the usual doses of application of treatments, that consists of a non-specific clinical picture, the so-called "thermal crisis” or “balneointoxication". Despite its clinical similarity (gastric discomfort, hepatic congestive outbreaks, cutaneous reactions, etc.) with that observed in acute cyanotoxin poisonings, thermal crisis has never been associated with the abundant growth of potentially toxic cyanobacteria in the mineral water sources. The aim of this work was to verify the hypothetical involvement of cyanotoxins in this clinical picture. Samples from mostly sulphurous water sources, with thermal characteristics ranging from cold to hyperthermal waters were analysed. ELISA (both in solution and in cellular matrix samples), LC-ESI-HRMS (in cellular matrix samples), and analysis of potential toxicity by means of a standardized bioassay were carried out. The toxic effect observed in the toxicity bioassays in one third of the sources may be related to the existence of microcystins and nodularins and even with other cyanobacterial peptides detected. In addition, several responses observed in the toxicity analyses reflect a pattern, probably linked to a type of hormetic response (hormesis is an adaptive response to low levels of stress, characterized by a biphasic dose-response curve).

A pulsating heat pipe embedded radiator: Thermal-vacuum characterisation in the pre-cryogenic temperature range for space applications

The use of Pulsating Heat Pipes (PHPs) in the space field is still an open issue because of the lack of data obtained during actual operation in relevant environment. A considerable amount of data is available in the literature on the thermal response of PHPs to a variable gravity condition and on the operation in the cryogenic field but in both cases the heat load at the condenser is rejected by convection to a constant temperature sink. On the other hand, barely any work in the PHP’s literature deals with thermal radiation to a low temperature sink as only heat transfer mode. The present work attempts to fill the gap by testing a PHP radiator (16 turns, 1.1 mm inner diameter, 50% filled with FC-72 at 293 K) in thermo-vacuum conditions in horizontal orientation at different heat loads and different environment (chamber) temperature. Fluid pressure measurements coupled with the frequency analysis characterised the effect of the cold source temperature on the device operational limits and efficiency. Results show that the device thermal performance in the radiative configuration is mostly affected by the lower operating temperatures needed to obtain a sensible heat rejection, rather than the heat transfer mode itself. The decrease of the environment temperature shortens the operational heat load range: the start-up occurs at higher heat input levels while the thermal crisis occurs at lower heat loads. The frequency analysis reveals that the equivalent thermal resistance is positively affected by higher values of the dominant frequency for all the cases.

Gas dynamic calculation of detonation in constant-cross-section ducts

The paper presents a computational method with the use of gas-dynamic functions of parameters of detonation in a one-dimensional subsonic flow of ideal gas behind the shock wave propagating in chemically active air-and-fuel mixture in a uniform-cross-section duct, where the resultant of normal pressure forces acting on its side surface is equal to zero. Stabilization of the shock wave is provided by the onset of thermal crisis with the air-and-fuel mixture combustion heat supply to the gas behind the wave. In this case the value of the specific speed of the combustion products is equal to the critical one. The solution of the total-impulse equation considering the above mentioned peculiarities of the flow in a uniform-cross-section duct establishes clear correlation of the specific speed of the stabilized shock wave to the rate of rise of the temperature of the gas behind it, which gives an opportunity to determine all detonation parameters. The shock wave can be initiated by the detonation of an explosive substance and carries a huge amount of energy. It is shown that the shock wave can be obtained only if the source of small disturbances itself moves at the supersonic speed. It is shown that total pressure behind the shock wave decreases significantly and the entropy increases due to the wave losses, whereas the static pressure increases significantly. An explanation of this effect is given. A formula for calculating the rate of gas temperature rise was derived as a function of the specific speed of the shock wave, the air-and-fuel mixture heat value and the heat availability factor that designates the dissociation of the combustion products and heat loss through the duct wall under specified initial conditions. The reliability of the method of calculating detonation was experimentally substantiated. The work is currently important for the evaluation of the detonation engine efficiency.

Can Wicking Control Droplet Cooling?

Wicking, defined as absorption and passive spreading of liquid into a porous medium, has been identified as a key mechanism to enhance the heat transfer and prevent the thermal crisis. Reducing the evaporation time and increasing the Leidenfrost point (LFP) are important for an efficient and safe design of thermal management applications, such as electronics, nuclear, and aeronautics industry. Here, we report the effect of the wicking of superhydrophilic nanowires (NWs) on the droplet vaporization from low temperatures to temperatures above the Leidenfrost transition. By tuning the wicking capability of the surface, we show that the most wickable NW results in the fastest evaporation time (reduction of 82, 76, and 68% compared with a bare surface at, respectively, 51, 69, and 92 °C) and in one of the highest shifts of the LFP of a water droplet (5 μL) in the literature (about 260 °C).

Gas dynamic calculation of detonation in variable cross-section ducts

Formulas of gas dynamic calculation of detonation parameters in variable cross-section ducts are presented and a design detonation diagram is given. The diagram shows the detonation characteristics of super-compressed detonation and under-compressed detonation as the function of shock wave specific speed depending on the intensity of temperature of the ideal gas in a subsonic one-dimensional flow behind the shock wave propagating in a chemically active air-fuel mixture and on the ratio of geometrical expansion (convergence) of the duct. The propagation of a stationary shock-wave the static pressure of which in the output cross-section of the expanded duct is equal to atmospheric pressure is referred to as design detonation. This means that all the energy of the shock wave at the output of the duct can be converted into polytropic work function of gas expansion in a detonation engine. Otherwise, if the flow takes place in the mode of overexpansion due to the separation of the compressive shock wave inside the duct or in the case of insufficient expansion part of the shock wave energy will be lost. The total impulse equation for a geometrically expanding duct is solved by replacing the integral describing the thrust force with the average integral value of the curve of the static pressure acting on the side wall of the expanding duct. The frictional force is neglected due to its insignificant value. It is shown that the presence of an insufficiently compressed shock wave is not possible as the shock wave moving at the supersonic speed in the convergent duct will be decelerated to the sonic speed. To stabilize it additional heat should be supplied to transform the convergent duct behind the compressive shock wave into a semi-permanent cross-section duct wherein thermal crisis stabilizing the shock wave can be achieved. The minimum value of the detonation pipe diameter of 50mm is substantiated. Below that value sharp reduction of combustion efficiency takes place. The results of the work can be used for the computation of detonation engine thermodynamic cycle parameters.

M-Oscillating: Performance Maximization on Temperature-Constrained Multi-Core Processors

The ever-increasing computational demand drives modern electronic devices to integrate more processing elements for pursuing higher computing performance. However, the resulting soaring power density and potential thermal crisis constrain the system performance under a maximally allowed temperature. This paper analytically studies the throughput maximization problem of multi-core platforms under the peak temperature constraints. To take advantage of thermal heterogeneity of different cores for performance improvement, we propose to run each core with multiple speed levels and develop a schedule based on two novel concepts, i.e., the step-up schedule and the m-Oscillating schedule , for multi-core platforms. The proposed methodology can ensure the peak temperature guarantee with a significant improvement in computing throughput up to 89 percent, with an average improvement of 11 percent. Meanwhile, the computational time reduces orders of magnitude compared to the traditional exhaustive search-based approach.

Assessment of the Subchannel Code CTF for Single- and Two-Phase Flows

As part of the Consortium for Advanced Simulation of Light Water Reactors, the subchannel code CTF is being used for single-phase and two-phase flow analysis under light water reactor operating conditions. Accurate determination of flow distribution, pressure drop, and void content is crucial for predicting margins to thermal crisis and ensuring more efficient plant performance. In preparation for the intended applications, CTF has been validated against data from experimental facilities comprising the General Electric (GE) 3 × 3 bundle, the boiling water reactor full-size fine-mesh bundle tests (BFBTs), the Risø tube, and the pressurized water reactor subchannel and bundle tests (PSBTs). Meanwhile, the licensed, well-recognized subchannel code VIPRE-01 was used to generate a baseline set of simulations for the targeted tests and solution parameters were compared to the CTF results.The flow split verification problem and single-phase GE 3 × 3 results are essentially in perfect agreement between the two codes. For the two-phase GE 3 × 3 cases, flow and quality discrepancies arise in the annular-mist flow regime, yet significant improvement is observed in CTF when void drift and two-phase turbulent mixing enhancement are considered. The BFBT pressure drop benchmark shows close agreement between predicted and measured results in general, although considerable overprediction by CTF is observed at relatively high void locations of the facility. This overestimation tendency is confirmed by the Risø cases. While overall statistics are satisfactory, both BFBT and PSBT bubbly-to-churn flow void contents are markedly overpredicted by CTF.The issues with two-phase closures such as turbulent mixing, interfacial and wall friction, and subcooled boiling heat transfer need to be addressed. Preliminary sensitivity studies are presented herein, but more advanced models and code stability analysis require further investigation.

Flow boiling heat transfer, dewetting-rewetting, and dryout visualization of HFOs in an asymmetrically heated rectangular plain channel

This paper presents the experimental measurements conducted during liquid and flow boiling heat transfer of R1234ze(E) and R1234yf in an asymmetrically heated rectangular plain channel. The tests were run by varying the refrigerant mass velocity from 50 to 200kgm−2s−1, the vapour quality from 0.2 to 1, and the heat flux between 50 and 100kWm−2, at a constant saturation temperature of 30°C. The results show that the nucleate boiling dominates the phase change process; furthermore, R1234yf shows higher two-phase heat transfer coefficients as compared to R1234ze(E), at the same operating test conditions. The developed measurement technique also permitted to estimate the vapour quality at the onset of the dryout. Furthermore, the direct visual observation of the boiling process allowed for a description of the phase-change and thermal crisis phenomena, including a detailed analysis of the dewetting-rewetting phenomenon.

A thermal margin preservation scheme for interactive multimedia consumer electronics

The heterogeneous multicore architecture is considered a cogent solution to match the performance demand for processing the next-generation media formats such as ultra-high definition, 3D or holography. However, the performance cores in a heterogeneous multicore processor dissipate a huge amount of heat. To cope with the thermal risk, most modern embedded processors provide the dynamic thermal management (DTM) feature that forcefully reduces the clock speed of the processors. Although this simple approach can maintain the system temperature below the thermal trip point, the performance of prioritized multimedia or interactive applications can be significantly harmed by the reduced performance even when the thermal crisis is caused mostly by the non-prioritized applications. This paper proposes a novel DTM scheme called Thermal Margin Preservation (TMP). TMP differentiates the thermal trip point for the prioritized applications from that for the nonprioritized ones, and thus forms the thermal margin, which is the temperature gap between the two trip points. Under the proposed scheme, the prioritized applications can run without any disturbance in the thermal margin by sacrificing the performance only of the non-prioritized applications. The evaluation shows that the proposed scheme significantly reduces the quality-of-service degradation for video playback under high temperature conditions.

Experimental Validation of Critical Heat Flux (CHF) Predictive Methods for a New Synthetic fluid with Low Environmental Impact

This work presents experimental critical heat flux (CHF) values for a low environmental impact synthetic refrigerant and their comparison with well-known correlations from scientific literature. Tests were performed with HFO-1234yf in an aluminum heat sink made up of seven mini-channels, each of them 2mm wide and 1mm high. The heated length was 25mm. Experiments have been obtained in a variety of thermodynamic condition: the R1234yf saturation temperatures Tsat ranged from 25 up to 65°C (corresponding to medium-to-high reduced pressures), whilst the mass fluxes G had been fixed to 150 up to 300kg/m2 s. The tests have been carried out by increasing the heat dissipated by the boiling refrigerant until the thermal crisis occurred and the corresponding heat flux value was recorded as CHF. The experimental results were finally compared to the well-known correlations of Wojtan et al. [1], Kuan [2], Katto-Ohno [3], Zhang et al. [4] and Anwar et al. [5] to investigate their effectiveness with the present data. The latter correlation was found to better predict the experimental values.

Lifetime Reliability Enhancement of Microprocessors

Ensuring lifetime reliability of microprocessors has become more critical. Continuous scaling and increasing temperatures due to growing power density are threatening lifetime reliability. Negative bias temperature instability (NBTI) has been known for decades, but its impact has been insignificant compared to other factors. Aggressive scaling, however, makes NBTI the most serious threat to chip lifetime reliability in today's and future process technologies. The delay of microprocessors gradually increases as time goes by, due to stress and recovery phases. The delay eventually becomes higher than the value required to meet design constraints, which results in failed systems. In this article, the mechanism of NBTI and its effects on lifetime reliability are presented, then various techniques to mitigate NBTI degradation on microprocessors are introduced. The mitigation can be addressed at either the circuit level or architectural level. Circuit-level techniques include design-time techniques such as transistor sizing and NBTI-aware synthesis. Forward body biasing, and adaptive voltage scaling are adaptive techniques that can mitigate NBTI degradation at the circuit level by controlling the threshold voltage or supply voltage to hide the lengthened delay caused by NBTI degradation. Reliability has been regarded as something to be addressed by chip manufacturers. However, there are recent attempts to bring lifetime reliability problems to the architectural level. Architectural techniques can reduce the cost added by circuit-level techniques, which are based on the worst-case degradation estimation. Traditional low-power and thermal management techniques can be successfully extended to deal with reliability problems since aging is dependent on power consumption and temperature. Self-repair is another option to enhance the lifetime of microprocessors using either core-level or lower-level redundancy. With a growing thermal crisis and constant scaling, lifetime reliability requires more intensive research in conjunction with other design issues.

Influence of the External Force on the Vortex–Source Combination Crisis

This paper considers the force stagnation of a supersonic vortex–source combination flowing out into a vacuum, leading to a decrease in the radial Mach number to unity and stationary flow choking. The similarity parameters influencing the crisis are analyzed. It has been shown that the ″force″ crisis (choking) is also possible for the vortexsink combination flowing out of a submerged space. The formulation of the thermal crisis problem is generalized to the case of a given external force per unit mass and in a unit volume. Comparison has been made between certain characteristics of the crisis due to the applied external force in the absence of power supply/removal and the characteristics of the crisis due to the heat input in the absence of external force. The investigation has been carried out from the viewpoint of the perfect gas model.

Determination of the Boundary Parameters of the Thermal Crisis of the Vortex-Source Combination at Energy Input with Constant Pressure

We have investigated the fl ow regions of the vortex-source combination with a constant pressure established due to the specially chosen intensity distribution of the energy exchange sources (heat input and heat removal) in the space of similarity parameters initial coordinate (temperature)–gas circulation–length of the energy exchange zone. Regions in which a thermal crisis is realized and steady fl ow choking occurs have been determined. We have constructed boundaries of the thermal crisis regions in the following cases of vortex-source combination outflow: into a vacuum, from a rarefied space, into a submerged space or from a submerged space.

Thermal crisis of a vortex source in a real gas flowing into vacuum with heat supply under constant pressure

Thermal crisis of a vortex source at energy input under constant pressure is investigated. The energy release intensities per unit mass and per unit volume are analyzed for the case of the vortex source flow into vacuum (regime I). Analytical solutions for gasdynamic parameters and an algorithm for constructing the heat release intensity function for different variants of vortex sources are studied. Limitations on the extension of a constant-pressure interval are determined, and the dependences of the energy input area critical coordinates, critical temperature, and energy input parameters on the mass flow, gas circulation, and initial coordinate of the energy input area are obtained. A small correction to the gas specific heat with rising temperature and a significant correction to the specific heat at a stagnation temperature of 1000 K are considered.

Effects of two-phase flow conditions on flow boiling CHF enhancement of magnetite-water nanofluids

Experimental studies were conducted to understand the flow boiling CHF (critical heat flux) characteristics of a magnetite-water nanofluid for a wide range of exit qualities, especially for intermediate and relatively high exit qualities. In contrast to previous studies, a wide range of exit qualities was considered for various heating lengths, mass fluxes, and inlet sub-cooling. CHF enhancement was observed when the exit quality was low and a DNB-like thermal crisis occurred: these results were consistent with the previous studies reporting a delay in DNB when using nanofluids. Meanwhile, this CHF enhancement did not occur for high exit qualities. In annular flow and high exit qualities, an LFD-type thermal crisis occurred. This CHF enhancement is not expected using a nanofluid under the LFD condition because LFD phenomena are nearly unaffected by the surface conditions. As the exit quality increased from 0.07 to 0.74, the amount of CHF enhancement gradually decreased and approached zero. In conclusion, this CHF enhancement using a nanofluid is not expected for annular flow with a relatively high exit quality.

Shock wave in the flow field of a source and a vortex source in the energy supply zone at thermal crisis

Thermal crisis of a vortex source outflowing initially in regime I into a rarefied space (into vacuum) with a transition of the supersonic flow into the subsonic flow in the shock wave, and into a stagnant space in regime II with final stagnation is considered in the model of a perfect gas with a constant heat capacity. The shock wave can be located in the energy supply zone or outside the energy release zone depending on the preset total pressure at infinity. In the absence of circulation, a cylindrical source is compared with a spherical source. The dependences of energy parameters and temperature, as well as the total pressure and density, on the coordinates of the shock wave are considered. The dependences of the critical parameters of the flow in the wake behind the zone on the coordinate of the heat supply zone, its length, and gas circulation in the cylindrical vortex source are analyzed.