Third heat‐transfer crisis with stepwise heat supply

The paper reports experimental results on heattransfer crises with stepwise heat supply to a heater, in which the metastable liquid decomposes in the form of vaporization fronts. Data on the dynamics of heattransfer crises under saturation and underheating conditions are given. It is shown that below the vapor formed during propagation of vaporization fronts, a liquid microlayer is absent.     

Forced convective heat transfer from premixed flames —Part 2: Impingement heat transfer

A study of convective heat transfer from impinging flames is completed with the presentation of heat transfer rates measured in premixed methane-air flames. Unburnt gas equivalence ratios from 0.8 to 1.2 have been examined, with burner exit Reynolds numbers ranging from 2000 to 12 000. Heat fluxes measured at the stagnation point of a body of revolution and a circular cylinder demonstrate that the trends observed in measured heat flux profiles are mainly determined by variations in the mean velocity and temperature within a flame, with peak heat transfer rates occuring within or close to the flame reaction zone. Increases in Reynolds number lead to an increase in the peak heat flux attained within a flame and to a decrease in the axial extent of the flame equilibrium region. Variations in equivalence ratio away from approximately stoichiometric conditions lead to a decrease in the maximum rate of heat transfer from a flame and to a shifting of the position of maximum flux downstream. Theoretical predictions applicable to the equilibrium region of the flames are in reasonable accord with experimental data.     

Effect of heat flow on heat‐transfer characteristics for a supersonic spatial flow around a spherically blunted cone

Heattransfer processes for a supersonic spatial flow around a spherically blunted cone were studied by solving direct and inverse threedimensional problems taking into account heat flow along the longitudinal and circumferential coordinates. It is shown that highly heatconducting materials can be used to advantage to decrease the maximum temperatures on the windward side of streamline bodies.     

Numerical modeling of pin–fin micro heat exchangers

A micro heat exchanger (MHE) can effectively control the temperature of surfaces in high heat flux applications. In this study, several turbulence models are analyzed using a 3D finite element model of a MHE. The MHE consists of a narrow planar flow passage between flat parallel plates with small cylindrical pin fins spanning these walls. The pin fin array geometry investigated is staggered, with pin diameters of 0.5, 5.1 and 8.5 mm, height to diameter ratio of 1.0 and streamwise (longitudinal) and spanwise (transverse) to diameter ratios of 1.5 and 2.5, respectively. Pressure loss and heat transfer simulated results for 4,000 ≤ Re ≤ 50,000 are reported and compared with previously published numerical and experimental results. It was found that the flat micro pin fin overall thermal performance always exceeds that of the parallel plate counterpart (smooth channel) by a factor of as much as 2.2 for the 8.5 mm diameter pins, and by 4 for the 0.5 mm diameter pins in the investigated Reynolds number range. Further, among the six turbulence models investigated, the RNG model tends to be the best model to predict both the Nusselt number and the friction factor and capture the main feature of the flow field in MHE.     

Melting heat transfer effects on stagnation point flow of micropolar fluid saturated in porous medium with internal heat generation （absorption）

The effect of melting heat transfer on the two dimensional boundary layer flow of a micropolar fluid near a stagnation point embedded in a porous medium in the presence of internal heat generation/absorption is investigated. The governing non-linear partial differential equations describing the problem are reduced to a system of non-linear ordinary differential equations using similarity transformations solved numerically using the Chebyshev spectral method. Numerical results for velocity, angular velocity and temperature profiles are shown graphically and discussed for different values of the inverse Darcy number, the heat generation/absorption parameter, and the melting parameter. The effects of the pertinent parameters on the local skin-friction coefficient, the wall couple stress, and the local Nusselt number are tabulated and discussed. The results show that the inverse Darcy number has the effect of enhancing both velocity and temperature and suppressing angular velocity. It is also found that the local skin-friction coefficient decreases, while the local Nusselt number increases as the melting parameter increases.     

Numerical study of turbulent flow and heat transfer of Al2O3–water mixture in a square duct with uniform heat flux

Turbulent flow and convective heat transfer of a nanofluid made of Al₂O₃ (1–4 vol.%) and water through a square duct is numerically studied. Single-phase model, volumetric concentration, temperature-dependent physical properties, uniform wall heat flux boundary condition and Renormalization Group Theory k-ε turbulent model are used in the computational analysis. A comparison of the results with the previous experimental and numerical data revealed 8.3 and 10.2 % mean deviations, respectively. Numerical results illustrated that Nu number is directly proportional with Re number and volumetric concentration. For a given Re number, increasing the volumetric concentration of nanoparticles does not have significant effect on the dimensionless velocity contours. At a constant dimensionless temperature, increasing the particle volume concentration increases the size of the temperature profile. Maximum value of dimensionless temperature increases with increasing x/D_h value for a given Re number and volumetric concentration.     

Effects of heat flux on λ-transition in liquid 4He

This paper is concerned with the derivation of a phase field model for λ-transition in ⁴He, when the liquid is subject to pressure and heat flux. As parameter that controls the transition, a field f that is the geometrical mean between the density of the fluid and that of the superfluid is used. The resulting model, that is a generalization of previous papers on the same subject, chooses as field variables the density, the velocity, the temperature and the heat flux, in addition to this field f. The restrictions on the constitutive quantities are obtained by using the Liu method of Lagrange multipliers. New results with respect to previous models are the presence of non-local terms to describe inhomogeneities in the field variables and dissipative effects of mechanical and thermal origin, and the fact that the model describes the behaviour of the liquid also far from the λ-transition region.     

Thermoelastic waves in helical strands with Maxwell–Cattaneo heat conduction

Harmonic thermoelastic waves in helical strands with Maxwell–Cattaneo heat conduction are investigated analytically and numerically. The corresponding dispersion relation is a sixth-order algebraic equation, governed by six non-dimensional parameters: two thermoelastic coupling constants, one chirality parameter, the ratio between extensional and torsional moduli, the Fourier number, and the dimensionless thermal relaxation. The behavior of the solutions is discussed from two perspectives with an asymptotic-numerical approach:(1) the effect of thermal relaxation on the elastic wave celerities, and(2) the effect of thermoelastic coupling on the thermal wave celerities. With small wavenumbers, the adiabatic solution for Fourier helical strands is recovered.However, with large wavenumbers, the solutions behave differently depending on the thermal relaxation and chirality. Due to thermoelastic coupling, the thermal wave celerity deviates from the classical result of the speed of second sound.     

Experimental investigations on a 1–2 heat exchanger with wire-wound tubes

This paper deals with experimental investigations on a 1–2 shell and tube heat exchanger, to study the effect of spiral turbulators on its performance. The heat exchanger has its tubes wound with copper wire, so that the winding acts as an augmenting device. Experiments were conducted with various winding wire diameters and pitches and the heat transfer coefficients were evaluated for a wide range of temperature levels and flow rates of the shell side fluid. The experimental results are discussed in detail and correlations are proposed to predict the shell side Nusselt number of the exchanger with varying winding pitches and diameters. The existence of optimum winding is also discussed in this paper. The present results are based on over 250 experimental observations made in the laminar range of flow.

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Experimentelle Untersuchungen an einem 1–2-Wärmetauscher mit drahtumwickelten Rohren

Zusammenfassung Diese Arbeit befaßt sich mit experimentellen Untersuchungen an einem 1–2-Zylindermantel/Rohrwärmetauscher und hat die Klärung des Einflusses von spiralförmigen Turbulenzpromotoren auf das Übertragungsverhalten zum Ziel. Die Rohre des Wärmetauschers sind mit Kupferdraht umwickelt, dessen Windungslagen den Austausch befördern. Die Experimente wurden mit verschiedenen Drahtdurchmessern und Steigungen durchgefürht und hieraus Wärmeübergangskoeffizienten in einem weiten Bereich der Temperaturniveaus und der Mengenströme des die Rohre umströmenden Fluids bestimmt. Die experimentellen Befunde werden eingehend diskutiert und Korrelationen zur Bestimmung der Nusselt-Zahl auf der Rohraußenseite in Abhängigkeit von Steigung und Durchmesser der Drahtwindungen angegeben. Die Ergebnisse basieren auf mehr als 250 Messungen im Bereich der Laminarströmung und belegen die Existenz einer optimalen Windungskonfiguration.

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Nomenclature A _o heat transfer surface area outside the tube, m² - A _i heat transfer surface area inside the tube, m² - d _o outer diameter of the tube, m - D inner diameter of the shell, m - D _e equivalent diameter of shell for heat transfer, m - d _w diameter of the winding wire, m - F _T LMTD correction factor [8] - h _i inside heat transfer coefficient, W/m² K - h _o outside heat transfer coefficient, W/m² K - k _o thermal conductivity of water outside tube, W/mK - L length of the shell, m - l _t tube length, m - l _w length of the winding wire, m - LMTD logarithmic mean temperature difference, K - m mass flow rate of cold fluid, kg/h - N number of tube passes - Nu _i inside Nusselt number for the tube - Nu _o outside Nusselt number for the tube - Nu _o * outside Nusselt number, average over the temperature range - n _t number of tubes per pass - P pitch of the winding wire, m - Pr _t Prandtl number, tube side - Q heat transfer rate, W - Re _t tube side Reynolds number - Re _s shell side Reynolds number for heat transfer - T corrected mean temperature difference, K [8] - Th _i inlet temperature of hot water, °C - Th _o exit temperature of hot water, °C - Ti _i inlet temperature of cold water, °C - Tc _o exit temperature of cold water, °C - U _o overall heat transfer coefficient, W/m² K     

Local heat transfer properties in co- and counter-current G-L-S magnetically stabilized fluidized beds

Heat transfer coefficients were measured by immersed probes in co- and counter-current G-L-S magneti-cally stabilized fluidized beds (MSFBs) using air, water and nickel-alloy particles as the gas, liquid and solid phases. Influences of major factors, including magnetic field intensity, superficial gas and liquid velocities, liquid viscosity and surface tension, on heat-transfer properties were studied experimentally, indicating that both co- and counter-current G-L-S MSFB can provide relatively uniform radi...     

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75^°. The effect of water to oil flow rate ratio (Q_wat/Q_oil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Q_wat/Q_oil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Q_wat/Q_oil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.     

Forced convective heat transfer from premixed flames: —Part 1: Flame structure

In this, the first part of a two-part study of convective heat transfer from impinging flames, the aerodynamic structure of four flames was studied. The flames examined were of stoichiometric mixtures of methane and air with a Reynolds number range extending from the laminar to fully turbulent flow regimes. Instantaneous Schlieren photographs revealed that with increasing Reynolds number the flame reaction zone extended further downstream and became thicker and more diffuse. Associated with this, measurements of mean and rms velocities and mean temperatures showed that the properties of the flames became drawn out in the downstream direction as Reynolds number increased. Schlierenstroboscopic techniques also revealed the existence of large scale vortex rings which originated in the shear layer of the flames, and which were found to cause low frequency oscillations in measured instantaneous velocities. These oscillations lead to misleadingly high levels of rms velocities downstream of the flame reaction zone which should not be interpreted as representing turbulence within the flame.     

The pressure drop characteristics of air–water bubbling flow for evaporative heat transfer

This paper presents a study on a novel water bubbling layer pressure drop and heat transfer experiment that was conducted to investigate the characteristics of pressure drop of air flow across the water bubbling layer. The attempt was to reduce the pressure drop while maintaining a higher value of the heat transfer coefficient. This type of heat transfer between water and merged tubes has potential application in evaporative cooling. To achieve the goal the pressure drop should be reduced by decreasing the bubble layer thickness through the water pump circulation. Pressure drops of air passing through the perforated plate and the water bubbling layer were measured for different heights of water bubbling layer, hole-plate area ratio of the perforated plate and the air velocity through the holes. Experimental data show that the increase of water bubbling layer height and air velocity both increase the pressure drop while the effect of the hole-plate area ratio of the perforated plate on the heat transfer coefficient is relatively complex. The measurements showed that even at a considerably lower height of water bubbling layer the heat transfer coefficient can exceed 5,000 W/m²-K. The heat transfer coefficients of 30 mm high water bubbling layer are higher than that of other higher water bubbling layers tested in the experiments     

Wiener–Hopf solution of rewetting of an infinite cylinder with internal heat generation

The two-dimensional quasi-steady conduction equation governing conduction controlled rewetting of an infinite cylinder with heat generation has been solved by Wiener–Hopf technique. The analytical solution yields the quench front temperature as a function of various model parameters such as Peclet number, Biot number and dimensionless heat generation rate. Also, the dry out heat generation rate is obtained by setting the Peclet number equal to zero, which gives the maximum permissible heat generation so as to prevent the dry out of the coolant.     

Unsteady heat transfer in impulsive Falkner–Skan flows: Constant wall temperature case

A theoretical study of the velocity and thermal boundary-layer growth resulting from an impulsively started Falkner–Skan flow is presented in this paper. The forced convection, thermal boundary-layer is produced by the sudden increase of the surface temperature as it is set into motion. Analytical solutions for the simultaneous development of the thermal and momentum boundary layers are obtained for both small (initial, unsteady flow) and large (steady-state flow) times. These solutions are then matched numerically using a very efficient finite-difference scheme. Some considerable attention to the steady-state flow solution (large time) is also given in this paper. Results of the calculations are presented for a range of values of the Falkner–Skan exponent m and the Prandtl number Pr.     

A heat transfer model for incorporating carbon foam fabrics in firefighter’s garment

In the present work, a numerical study was performed to predict and investigate the performance of a thermal protection system for firefighter’s garment consisting of carbon foam fabric in both the outer shell and the thermal liner elements. Several types of carbon foam with different thermal conductivity, porosity, and density were introduced to conduct a parametric study. Additionally, the thickness of the introduced carbon foam fabrics was varied to acquire optimum design. Simulation was conducted for a square planar 2D geometry of the clothing comprising of different fabric layers and a double precision pressure-based implicit solver, under transient state condition was used. The new anticipated thermal protection system was tested under harsh thermal environmental conditions that firefighters are exposed to. The parametric study showed that employing carbon foam fabric with one set of designed parameters, weight reduction of 33 % in the outer shell, 56 % in the thermal liner and a temperature reduction of 2 % at the inner edge of the garment was achieved when compared to the traditional firefighter garment model used by Song et al. (Int J Occup Saf Ergon 14:89–106, 2008). Also, carbon foam fabric with another set of designed parameters resulted in a weight reduction of 25 % in the outer shell, 28 % in the thermal liner and a temperature reduction of 6 % at the inner edge of the garment. As a result, carbon foam fabrics make the firefighter’s garment more protective, durable, and lighter in weight.     

Flow and heat transfer of a slightly rarefied gas over a stretching surface

This work deals with the study of the boundary layer flow and mass transfer of a visco-elastic fluid immersed in a porous medium over a stretching surface in the presence of surface slip, chemical reaction and variable viscosity. The partial differential equations governing the flow have been transformed by similarity transformation into a system of coupled nonlinear ordinary differential equations which is solved numerically by means of the fourth order Runge-Kutta integration scheme coupled with the shooting technique. The effects of various involved interesting parameters on the velocity fields and concentration fields are shown graphically and investigated. In addition, tabulated results for the local skin-friction coefficient and the local Sherwood number are presented and discussed.     

Transient heat conduction in spheres for Fo < 0.3 and finite Bi

Using Laplace transforms, an analytical solution was obtained for transient heat conduction in spheres exposed to surroundings at a uniform temperature and finite Bi numbers. The solution is explicit, and valid during early transients, for Fourier numbers Fo < 0.3