Heat Transfer Instabilities Research Articles

Startup characteristics of the large length–diameter ratio high-temperature heat pipe

Large length–diameter ratio high-temperature sodium heat pipes (HTSHPs) are uniquely structured heat-transfer devices with significant application potential in high-temperature solar thermal power and small-scale nuclear systems. This study tested HTSHPs' startup characteristics examining various parameters. Results indicate that the transition temperature for flow transition is 564 K, and the continuous flow transition temperature is 722 K. Heat pipes at 15 % fill ratio are adaptable across various inclination angles, whereas those at 25 % and 35 % fill ratios show optimal startup at smaller angles (15°, 30°), with decreasing startup time as angle increases, notably significant at 45°.The start-up time refers to the duration from the initiation of heating to the point when all the normal operational temperature points of the HTSHP reach the continuous flow transition temperature during the condensation section. During the experiments, heat pipes with a large length–diameter ratio exhibit heat transfer instability, temperature oscillation and local overheating. Reducing the fill ratio and inclination angle effectively reduces the probability of temperature oscillation and local overheating. This study proposes a novel method for optimizing temperature oscillations in heat pipe design, offering pioneering insights and experimental data crucial for specialized high-temperature heat pipe research.

Effect of three types of magnetic field modulation on the instability of heat transfer in micropolar nanofluid filled within Hele-Shaw cell

This article investigates the impact of three different types of magnetic field modulation on the instability of micropolar nanofluid filled within Hele-Shaw cells applying both nonlinear and linear approaches. The truncated Fourier series approach is used to analyze nonlinear stability, while the analytical evaluation of linear stability is performed using the normal mode methodology. All the results are graphically demonstrated. Based on the outcomes, the micropolar parameter, magnetic Chandrasekhar number, Hele-Shaw number, and the coefficient of coupling between vorticity and spin effect have stabilized effects in the system. As opposed to that, nanoparticle Rayleigh number accelerate the start of convective motion inside the system. Some parameters play a significant influence in the transport of heat and mass in nonlinear analysis. The Hele-Shaw number, micropolar parameter, magnetic Prandtl number, and magnetic Chandrasekhar number perform a major contribution to the heat/mass transfer in the system. Using two different approaches, the system’s heat and mass transmission are investigated, the Runge–Kutta–Fehlberg method and Mathematica NDSolve, and It has been found that the rate of heat/mass transfer in both conditions is exactly the same. Moreover, It has been found that heat and mass transmission start earliest in the case of triangle waveforms as compared to other types of magnetic field modulations.

Flow and Heat Transfer Instability of Supercritical Carbon Dioxide in a Vertical Heated Tube

Flow and Heat Transfer Instability of Supercritical Carbon Dioxide in a Vertical Heated Tube

Deterioration and Instability of Heat Transfer to H2O/CO2 Mixtures in Near-Critical Region for Horizontal Flow

Abstract In this work, we experimentally investigated the deterioration and instability of heat transfer to H2O/CO2 mixtures in the near-critical region, following our series of works on their normal and enhanced heat transfer, hydraulic resistance, and thermophysical properties. The experimental pressure is 24 MPa, temperatures are 330–400 °C, CO2 mass fractions are 9.8 and 15.0%, mass fluxes are above 940 kg/m2/s, heat fluxes are below 211 kW/m2, and flow is horizontal. Experiments show that the heat transfer deterioration for the supercritical H2O/CO2 mixture occurs when the fluid temperature is lower than the pseudo-critical temperature (Tpc) and the wall temperature is higher than Tpc, similar to the condition for supercritical pure fluids, but the onset heat flux is much lower. Results also show that the heat transfer instability for the supercritical H2O/CO2 mixture occurs not only moderately near Tpc (similar to supercritical pure fluids) but also more significantly near the critical miscible temperature. All of these distinct phenomena were attributed to the mixture feature.

Experimental study on influence of forced vibration of cooling channel on heat transfer instability at atmospheric and quasi-critical pressure

Hydrocarbon fuel is used as coolant to cool scramjet by flowing through cooling channels at atmospheric pressure and quasi-critical pressure conditions. The instability of the heat transfer will occur in this process. However, the effect of scramjet vibration on the heat transfer instability is unclear. In order to study the effect of cooling channel forced vibration on the unstable heat transfer performance at trans-crtical pressure, cooling channel heat transfer characteristics under different vibration condition are analyzed. Experimental results show that at atmospheric pressure, cooling channel vibration causes a drastic change in the temperature of the inner wall during unstable heat transfer process, but vibration will not change the fuel bulk temperature oscillation process. As a result, forced vibration can lead to heat transfer deterioration in the gas-liquid two-phase flow. Under the condition of quasi-critical pressure, cooling channel vibration not only change the inner wall temperature, but also influence the fuel bulk temperature. The forced vibration can lead to heat transfer enhancement. High frequency vibration can effectively suppress heat transfer instability and reduces heat transfer fluctuations.

Analysis of supercritical heat transfer in horizontal helical tube with internal roughness

• Supercritical flow in horizontal helical tube with internal roughness is analyzed. • Circumferential non-uniformity and axial oscillation of T w are explored. • The internally ribbed roughness and concaved roughness are compared. • The effects of spiral roughness, centrifugal force, and buoyancy are revealed. Helically coiled tube with smooth wall (HSW) has been widely used in the fields of supercritical air conditioning systems and supercritical heat pump systems due to its compact structure and excellent heat transfer performance. However, its large circumferential non-uniformity and intense axial oscillation will bring to local stress concentration and instability of heat transfer which are both responsible for system accidents. To alleviate the above two crises, the spiral inner wall roughness is proposed to be introduced into HSW. Up to now, the performance of helically coiled tube with internally ribbed roughness (HRW) has been seldom reported and the effectivenesses of inner roughness with different structures on suppressing circumferential non-uniformity and axial oscillation are still unclear. In this work, the SST k - ω model is employed to simulate the turbulent flow of s-R1234ze(E) in horizontal HRW to fill these gaps. Firstly, the effectivenesses of different rib heights, rib lift angles, rib widths, rib numbers, and rib base angles on restraining circumferential non-uniformity and axial oscillation are explored. The influences of wall thickness are also contained. Subsequently, the internally ribbed roughness and concaved roughness are compared and evaluated according to the overall comprehensive performance criterion. Finally, the effects of spiral roughness, centrifugal force, and gravitational buoyancy force on the flow field distributions are also revealed.

Natural convective heat transfer investigation of nanofluids affected by electrical field with periodically changed direction

Nanofluids with excellent thermal properties have significant advantages in the heat transfer enhancement. While, nanoparticles are easily deposited on the heat transfer interface or electrode after long-term running, which causes serious heat transfer instability. Therefore, the resuspension and natural convective heat transfer of nanofluids affected by electrical field with periodically changed direction were studied in the present study. Specifically, the variation of deposited nanoparticle layer on heat transfer surface was observed. Meanwhile, the resuspended nanofluid heat transfer characteristics were analyzed. Furthermore, mechanism of nanoparticle resuspension and heat transfer performance of resuspended nanofluid were also discussed. It is found that nanoparticles could be resuspended stably by electrical field with periodically changed direction. Meanwhile, the heat transfer performance of resuspended nanofluid under electrical field is significantly enhanced when compared to the pure base fluid, which is increased with the improvement of voltage, concentration, and direction switching time. The mechanism analysis shows that the deposited nanoparticles are resuspended by the driven of upward electric forces and drag of basefluid, and limit in a certain range by the periodically changed electrical field. Besides, A dimensionless heat transfer prediction model of nanofluid affected by electrical field with periodically changed direction was proposed, which has an acceptable reliability.

Experimental analysis on mechanism and prediction of thermo-acoustic instability of supercritical RP-3 fuel flowing in a vertical upward channel

This article presented an experimental investigation of thermo-acoustic instability of RP-3 (Rocket Propellant 3) aviation kerosene under supercritical pressures in a vertical upward circular channel. The oscillation characteristics of measured parameters were studied. The mechanisms and characteristics of unstable flow and heat transfer were analyzed. The effects of operating parameters, including the inlet fluid temperature, inlet fluid pressure, mass flow rate, and heat flux, on flow and heat transfer instabilities were discussed. The investigation on boundary between stable and unstable conditions under various operating parameters was performed, and two new empirical correlations for critical heat flux prediction were obtained. Results indicate that the coupling relation between the inside-wall temperature, outlet fluid temperature, and inlet-outlet pressure drop is found. The instability of pseudo-film boiling leads to this unstable phenomenon. The critical heat flux increases as the inlet fluid pressure rises, while increases first and then decreases with the increasing Reynolds number of inlet fluid. The proposed correlations work reasonably well in predicting the onset of thermo-acoustic instability.

Investigation of flow and heat transfer instabilities and oscillation inhibition of n-decane at supercritical pressure in vertical pipe

The characteristics of the flow and heat transfer instabilities of n-decane at supercritical pressures were evaluated under different inlet temperature, pressure, mass flow, and flow direction conditions. The experiment was carried out in a vertical heating tube with an inner diameter of 2 mm, at a maximum heat flux density of 200 kW/m2, under both laminar and turbulent inlet conditions. The results revealed that an increase in the inlet temperature, pressure, or mass flow rate can weaken the flow and heat transfer oscillation. Two oscillation stages of were observed, one is the transition oscillation that occurs when the fluid flows from laminar to turbulent, the other is the Helmholtz oscillation due to the uneven distribution of the radial density and the density distribution in the flow direction. Furthermore, a novel oscillation inhibition method was proposed, which involves the insertion of a twisted stainless-steel wire from certain parts of the heating tube. The influence of the insertion methods on the heat transfer enhancement and oscillation inhibition was evaluated using a comprehensive heat transfer coefficient. The results revealed that the insertion of a twisted wire from the tube inlet was more effective for the inhibition of the oscillation than that from the tube outlet. The insertion of a twisted wire into the entire length of the tube resulted in the best comprehensive performance of heat transfer and flow resistance when compared with those of other insertion methods.

Instability during transition to turbulence of supercritical pressure CO2 in a vertical heated serpentine tube

Convection heat transfer of supercritical pressure CO2 in various channels has extensive applications in many industries. In some applications, the channel may not be straight, but rather serpentine. Researchers have found that the transition to turbulence of constant-property fluid is delayed in a constant-curvature pipe, whereas it is promoted in a serpentine tube with alternating curvature. In supercritical pressure CO2 flow in a serpentine tube, the buoyancy and centrifugal forces both influence the transition process, which needs to be studied thoroughly. This study investigates the instability during transition from laminar to turbulent flow both numerically and experimentally. The wall temperature oscillations are measured by experiments. The effects of buoyancy and centrifugal force on the flow and heat transfer are analyzed through comparisons of unheated cases, to heated cases of forced flow (no buoyancy), upward flow and downward flow by simulations. The flow and heat transfer instabilities along the flow and across the cross sections and the secondary flow pattern development are studied. The location (distance from the inlet) at which the flow becomes unstable is least for downward flow, intermediate for upward flow and largest for pure forced flow. It is found that the earlier occurrence of the instability is caused by the adverse pressure gradient in downward flow, and the enhancing effect of the buoyancy on the centrifugal force in upward flow. These findings are important for understanding the flow and heat transfer during transition to turbulence of supercritical pressure CO2 flow in vertical serpentine tubes.

A Discussion on the Limits to Coating Reproducibility Based on Heat Transfer Instabilities

During the past several decades, plasma spraying has emerged as an important rapid solidification technology, aimed to depositing protective and functional coating on a variety of substrates in sectors as diverse as aerospace, manufacturing, automotive, consumer, and biomedical industries. As these applications demand high-quality coatings with reproducible and reliable properties, stringent process control is mandatory, supported by sophisticated diagnostic equipment. However, owing to the highly nonlinear nature of the plasma spray process, non-deterministic coating property changes make these requirements difficult if not impossible to achieve. In this contribution, possible reasons for nonlinearity are discussed based on a stability theory formalism as described by a cusp catastrophe of codimension 2 (Riemann–Hugoniot catastrophe).

ALGORITHM FOR THE PROCESS OF THAWING-FREEZING SOIL REGULATING AROUND THE GROUND PIPELINE IN PERMAFROST

In the design of the main and field pipelines in permafrost areas issues of methods of laying and operating conditions selection of the pipeline need to be solved. Engineering activity leads to the destruction of land cover, which in areas with permafrost contributes to the emergence and development of thaw- ing halo or trenches filled with water with a sharp increase in the sea- sonal thaw depth, activation of thermokarst, thermal erosion and other cryogenic processes. Thawing of permafrost soils causes irreversible waterlogging of the main territory, irreversible environmental damage. The current conditions for the development of these regions do not contribute to the preservation of frozen soils, and they are rapidly degrading. The reliability and durability of the structures are the higher the less disturbed cryogenic natural clay base state. This can be achieved by using ground pipelining method. The design of pipeline systems in these areas is particularly difficult due to the inherent instability of heat transfer and thermodynamic processes occurring in the soil. This article shows that the increase in environmental safety and health at the terrestrial trunk pipeline laying is possible by adjusting the halo thawing and thermal-selection mode pumping oil. Ground pipeline heat transfer problem during summer and winter operation taking into account the regulation halo thawing is solved. Its dimensions depend on the magnitude of the heat flow coming from the pipe into the ground. The algorithm for thermohydraulic pumping mode selection, during which the magnitude of halo thawing stays within acceptable limits dur- ing the entire period of operation is proposed. By controlling the thawing halo around the tube, the hydraulic mode stability is achieved with minimum oscillations of pumping performance and temperature during the annual period of use. The practical value of this work is in the fact that the resulting calcula- tions can be used in formulating and solving various technical and eco- nomic problems, taking into account the requirements of environmental integrity, and the search for optimal solutions in the design of pipelines in permafrost.

Experimental investigation on flow and heat transfer instabilities of RP-3 aviation kerosene in a vertical miniature tube under supercritical pressures

This paper presents experimental investigations on flow and heat transfer instabilities of RP-3 aviation kerosene under supercritical pressures in a vertical tube with inner diameter of 2 mm. The flow changes from laminar to the transition regime as the fuel is heated, which resulted from decrease of fluid density and increase of velocity. A 13-component surrogate model of RP-3 aviation kerosene is used to calculate the thermophysical properties of fuel. Under large wall heat flux conditions, instabilities in flow and heat transfer are observed when the inner wall temperature and outlet fluid temperature approach the pseudo-critical temperature, respectively. The former is accompanied with enhancement in heat transfer and could be explained by the pseudo-boiling mechanisms. Thermally-induced pressure oscillations due to drastic density variation is found to be the reason for the latter case. The effects of inlet pressure, wall heat flux and inlet Reynolds number on flow and heat transfer instabilities are also studied. It is found that higher inlet pressures and heat fluxes can weaken the flow instabilities. The influence of inlet Reynolds number is more complicated, different types of flow instabilities could be observed with the increase of inlet Reynolds number. In addition, stability boundaries are obtained based on the experiments data, which could be used to quantitatively predict the occurrence of flow and heat transfer instabilities.

Experimental investigation of the flow and heat transfer instabilities in n-decane at supercritical pressures in a vertical tube

The fuel may be used in hypersonic vehicles to cool various surfaces; however, supercritical pressure fuels have been found to have deteriorated heat transfer rates and instabilities for some flow conditions. The flow and heat transfer instabilities in supercritical pressure n-decane were investigated experimentally in this work, with pressure of 2.5 and 3.0 MPa and inlet temperature of 16–225 °C. The heat flux was slowly increased to observe the flow in different stages as well as to obtain the boundary lines for a stability map. Seven stages were observed with different stability features. The transition to turbulence was found to be the main reason for the instability for stage b with slightly irregular oscillations, while dramatic variations of the thermal properties caused Helmholtz oscillations with regular frequencies and large amplitudes. The heat transfer deterioration in conjunction with an instability with buoyancy due to the density variation was found to be the reason. Higher pressures, inlet mass flow rates or fluid temperatures or downward flow weakened the instabilities, so these should be used in engineering designs to reduce the heat transfer deterioration and instability. The stability map provides further support for the nonlinear dynamic theory explaining the oscillations.

Experimental study on saturated flow boiling heat transfer of nitrogen in a small-diameter horizontal heated tube

In this study, experiments were performed to study the saturated flow boiling heat transfer characteristics of saturated nitrogen in a horizontal stainless steel heated tube with an internal diameter of 2.92mm and length of 1000mm. The heat transfer coefficients (1134 data points) were obtained under a wide range of operation conditions (109 cases), e.g. heat flux of 1.4–43.7kW/m2, mass flux of 170–310kg/(m2s), inlet pressure of 192–350kPa, and vapor quality of 0.0015–0.71. Results showed that nucleate boiling dominated the heat transfer mechanism in the small-diameter horizontal tube under the conditions in the tests. The heat transfer coefficients had little dependence on mass flux or vapor quality but had strong correlation with heat flux and working pressure. Instability of heat transfer was also observed in the experiments when the vapor quality is greater than 0.2. In the experiments, the temperature oscillations show a relatively strong relationship with vapor quality rather than heat flux, suggesting the flow pattern change from steady bubble or plug flow to unsteady annular flow or intermittent local dry out when vapor quality is about 0.2. The correlation of Tran et al. can give predictions in good agreement with the experimental data, and therefore it is recommended in predicting heat transfer coefficient of saturated nitrogen flow boiling in small-diameter horizontal tubes.

Fluid flow and heat transfer characteristics of separation and reattachment flow over a backward-facing step

In this study, a direct numerical simulation of the fluid flow and heat transfer characteristics of separation and reattachment flow at a backward-facing step is presented. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the Reynolds number and expansion ratio on the fluid flow and heat transfer characteristics are investigated. The size of the primary recirculation zone increases with the reduction of expansion ratio and the fluctuation of isotherms increased with the increase of Reynolds number. The periodic characteristics and the dissimilarity between Nu and Cf appear in the transitional flow regime. The rotating fluids in the reattachment region increase the flow instability and the interchange of the hot and cold fluids increases heat transfer instability. The combined effects of flow instability and heat transfer instability play an important role in the formation of the dissimilarity between Nu and Cf.

Flow boiling heat transfer of water in microchannel heat sink

The flow boiling heat transfer of water in a microchannel heat sink with variable initial vapor quality at the inlet is investigated. The stainless steel microchannel heat sink contains ten 640 × 2050 µm channels with a length of 120 mm; the wall roughness is 10 µm. The data on the local heat-transfer coefficient distribution in heat sink length are obtained in the range of mass fluxes from 30 to 90 kg/m2s, heat fluxes from 40 to 170 kW/m2, and vapor qualities from 0 to 1. The heat transfer instability associated with dry spots resulting from insufficient wetting of channel walls introduces substantial contribution to the heat transfer mechanism and leads to decreasing heat transfer in heat sink length downward the flow. The developed method for calculating the flow boiling heat transfer of water in a microchannel heat sink allows more accurate prediction of heat transfer drop than available methods.

Special Features in the Realization of Film Boiling on Wire Heaters. Distilled Water

The existence of a region of self-oscillatory instability of heat transfer is revealed under conditions of film boiling of water subcooled to the saturation temperature. In the case of short heaters, whose length is commensurable with the Taylor instability wavelength, the process is regular. For heaters of the same length, the form of oscillations and their characteristics are fully reproducible. An increase in the heater length results in the emergence of spatially distributed oscillations. It is found that an increase in subcooling on the interface between the modes of boiling is accompanied by the formation of a structure consisting of a system of single domains of continuously decreasing size. The formation of this structure causes a significant change in the temperature distribution in the zone adjoining that of nucleate boiling, which considerably reduces the film stability. A single domain of the film mode of boiling was stable at any temperature of the liquid. The singularities of realization of minimal-size domains of the film mode of boiling under conditions of stabilization of the integral mean temperature of the heater are investigated. It is found that the minimal zone of the film mode of boiling is a system of three elementary domains.

Analysis Of The Heat Transfer Instability In A Wavy Channel For A Developing Regime

Analysis Of The Heat Transfer Instability In A Wavy Channel For A Developing Regime

Instability of heat transfer from HTSC-samples to liquid nitrogen

The heat transfer characteristics on the HTSC-surface in a pool of liquid nitrogen under conditions of slow increasing or decreasing transport current have been experimentally studied. It was observed that there are instabilities of heat transfer regimes regarding the small thermal disturbances (fluctuations). These unstable heat transfer regimes are accompanied by large temperature overshoots and, therefore, must be taken into consideration in designing HTSC-elements of superconducting systems.