•Geyser boiling can eventually yield intense evaporator accelerations in loop-thermosyphons.•Acceleration amplitudes up to 110 and 1100m/s2 were observed for filling ratios of 0.5 and 0.9, ...respectively.•In thermosyphon unsteady regime, geyser occurs for heat fluxes less than 20kW/m2 and vapor pressures less than 25kPa.•In thermosyphon steady-state regime, geyser occurs for heat fluxes higher than 12.5kW/m2 and vapor pressures below 25kPa.
Geyser boiling is experimentally investigated in two-phase closed loop-thermosyphons, consisting of two parallel condensers and a shared evaporator. Heat sink conditions at each condenser vary from forced to natural convection in a multitude of thermal arrangements. A cartridge resistance provides input power ranging from 0.1 to 0.85kW to the evaporator. Water is employed as working fluid with filling ratios of 0.5 and 0.9. The effects of thermal conditions in both condensers, filling ratio, heat flux and vapor pressure on geyser boiling phenomenon are investigated. Geyser boiling eventually yields intense evaporator vibrations inferred by acceleration measurements. The ratio of convective thermal resistances acting at each condenser affects the acceleration. Amplitudes up to 110 and 1100m/s2 were observed for filling ratios of 0.5 and 0.9, respectively. In unsteady regime, geysering occurs for heat fluxes less than 20kW/m2 and vapor pressures less than 25kPa. The vapor pressure is increased with increasing heat flux, suppressing geyser boiling intensity. In steady-state regime geyser boiling occurs for heat fluxes higher than 12.5kW/m2 and vapor pressures below 25kPa.
•Mechanisms for causing flow instabilities in pseudo-potential LB models were analyzed.•Numerical instability is caused by inappropriate inter-particle force and equation of state.•Universal methods ...of using limiter and sign functions were proposed to improve stability.•Higher density ratio & Re, lower temperature, and reduced spurious velocity are obtained.•The methods extend pseudo-potential LB model and are demonstrated by droplet splashing.
Since the pseudo-potential lattice-Boltzmann (LB) model was proposed, it has been suffering from low density ratio, small Reynolds number and flow instabilities, which hamper its application in many science and engineering problems. In this work, we analyze the reasons of flow instabilities in the pseudo-potential LB models, and propose some methods to improve the simulation stability. It is shown that the inter-particle interaction force term and the equations of state (EOS) can result in numerical instability in the particle distribution functions and density. Some straightforward and universal techniques are proposed here in order to achieve larger density ratio, higher Reynolds number and lower temperature as well as suppressing spurious velocity in multiphase flow in the pseudo-potential LB models without additional influences on the equilibrium properties in most cases. These methods contribute to extending the pseudo-potential LB models to realistic multiphase flow further. Finally, we demonstrate the method application for droplet splashing with Re = 15000, We = 120 and density ratio = 4792 at 0.45Tc successfully.
•Investigation of single-phase heat transfer under exponential power inputs.•Heat transfer regime transitions are related to the penetration depth of the thermal boundary layer.•Predicting transient ...heat transfer requires knowing the diffusive property of the turbulent boundary layer.•Analytic method to predict the Nusselt number under exponential power inputs in fully developed flows.
We present an experimental and theoretical investigation of single-phase heat transfer under exponential power inputs. We conduct forced flow experiments with water, covering a broad range of mass fluxes (from 0 to 19,300 kg/m2/s), bulk temperatures (from 25 to 100 °C), pressures (from 0.1 to 1.2 MPa), exponential power escalation periods (from 2.5 to 200 ms), and considering two different heater and channel geometries. We use a high-speed infrared thermometry technique to measure the space- and time-dependent heat transfer coefficient between the heated surface and the coolant, building a database covering 73 different experimental conditions. We consider turbulence as a diffusive process and develop an analytic model that can predict 80% of our database within a ±10% error, and the entire database within a ±20% error. We discuss the presence of three heat transfer regimes, i.e., transient conduction, transient turbulent diffusion, and quasi-steady turbulent heat transfer, and derive analytically the two associated transition criteria. These transitions depend on the power escalation period, fluid properties, and are connected to the profile of the turbulent diffusion properties across the boundary layer.
•Six heat transfer regimes identified.•Existence of a turbulent flow but laminar-like heat transfer regime for low Prandtl numbers.•Prediction of Nusselt number in turbulent flow and turbulent heat ...transfer regime.
This paper investigates heat transfer regimes in fully developed plane channel flows without considering the buoyancy effect. Analyzing the governing thermal energy equation, aided by direct numerical simulation (DNS) data, six heat transfer regimes are identified including (i) laminar flow and laminar heat transfer (LamF-LamH), (ii) transitional flow and laminar-like heat transfer (TraF-LamH), (iii) transitional flow and transitional heat transfer (TraF-TraH), (iv) turbulent flow but laminar-like heat transfer (TurF-LamH), (v) turbulent flow and transitional heat transfer (TurF-TraH), and (vi) turbulent flow and turbulent heat transfer (TurF-TurH). One key result is the clarification of a TurF-LamH regime which exists only for low Prandtl number fluid (Pr≪1). A critical non-dimensional number is determined as ReτPr1/2≲50 where Reτ is the Reynolds number defined by the channel half-height δ and the frictional velocity uτ. In the TurF-LamH regime, the simplified thermal energy equation yields a prediction of Nusselt number as Nu≈6.0. The clarification of the TurF-LamH regime provides valuable insight into the understanding of the Nusselt number data for liquid metals, which have very low Prandtl number. Another major finding is that, in the TurF-TurH regime, the Kader-Yaglom-style equation is shown to be better than the traditional power law correlations at predicting Nusselt number, for either low or high Prandtl numbers. No separate correlations are needed for the prediction of Nusselt number at low Prandtl numbers and high Prandtl numbers. Another advantage of the Kader-Yaglom-style equation is that the equation can be theoretically connected to the log-law for the mean velocity and the mean temperature. More importantly, in the TurF-TurH regime the prediction of Kader-Yaglom-style equation can be reliably extended to ultra high Reynolds numbers, for either low or high Prandtl numbers.
Impinging jets provide a means of achieving high heat transfer coefficients both locally and on an area averaged basis. The current work forms the first stage of a two part investigation of heat ...transfer distributions from a heated flat surface subject to an impinging air jet for Reynolds numbers from 10,000 to 30,000 and non-dimensional surface to jet exit spacing,
H/
D, from 0.5 to 8. In the present paper, the relative magnitudes of the local heat transfer coefficients are compared to the fluctuating components and to the mean and root-mean-square local velocity components. It has been shown that at low nozzle to surface spacings (<2 diameters) secondary peaks in the radial heat transfer distributions are due to an abrupt increase in turbulence in the wall jet. In particular the velocity fluctuations normal to the impingement surface have a controlling influence on the enhancement in the wall jet.
This study utilizes experimental analysis with window program VCTM V1.0 to investigate the thermal performance of the vapor chamber and apply to 30
Watt high-power LEDs. The thermal experiment method ...is derived a novel empirical formula for the effective thermal conductivity of the vapor chamber and calculated its thermal performance. Results show that the maximum effective thermal conductivity is 870
W/m
°C from the novel empirical formula, and comparing it with the experimental value, the calculating error is no more than ±5%. And the LED vapor chamber-based plate works out hot-spot problem of 30
Watt high-power LEDs, successfully.
The possibility of modeling the Navier–Stokes equations and together with the conventional second order slip boundary condition at high Knudsen numbers is explored in this paper by incorporating the ...Knudsen diffusion phenomenon in rarefied gases. An effective mean free path (MFP) model is augmented to the governing equation and the slip boundary condition, as gas transport properties can be related to the MFP. This simple modification is shown to implicitly take care of the complexities associated in the transitional flow regime, without necessitating dependency of the slip coefficients on the Knudsen number. Unique analytical model with fixed values of slip coefficients is proposed and rigorous comparisons with the experimental and simulation data for pressure driven and thermally driven rarefied gas flows support this conjecture. First and second order slip coefficients have been proposed as 1.1466 and 0.9756 for rectangular channels and 1.1466 and 0.14 for the capillaries, from the continuum to the transition flow regime. The current work is significant from the numerical simulation point of view because simulation tools are better developed for Navier–Stokes equations.
In this paper, meshless element free Galerkin (EFG) method has been extended to obtain the numerical solution of nonlinear, unsteady heat transfer problems with temperature dependent material ...properties. The thermal conductivity, specific heat and density of the material are assumed to vary linearly with the temperature. Quasi-linearization scheme has been used to obtain the nonlinear solution whereas backward difference method is used for the time integration. The essential boundary conditions have been enforced by Lagrange multiplier technique. The meshless formulation has been presented for a nonlinear 3-D heat transfer problem. In 1-D, the results obtained by EFG method are compared with those obtained by finite element and analytical methods whereas in 2-D and 3-D, the results are compared with those obtained by finite element method.
•Review of fundamental and frontier research of flow boiling in microscale channels.•Analysis of mechanisms of microscale channel flow boiling heat transfer.•Evaluation of flow boiling heat transfer ...models and correlations with a database.•Flow pattern based flow boiling heat transfer models in microscale channels.•Brief on unstable and transient flow boiling heat transfer in microscale channels.
This paper presents state-of-the-art review on the fundamental and frontier research of flow boiling heat transfer, mechanisms and prediction methods including models and correlations for heat transfer in microscale channels. First, fundamental issues of current research on flow boiling in microscale channels are addressed. These mainly include the criteria for macroscale and microscale channels. Then, studies on flow boiling heat transfer behaviours and mechanisms in microscale channels are presented. Next, the available correlations and models of flow boiling heat transfer in microscale channels are reviewed and analysed. Comparisons of 12 correlations with a database covering a wide range of test parameters and 8 fluids are presented. It shows that all correlations poorly agree to the database. No generalized model or correlation is able to predict all flow boiling heat transfer data. Furthermore, comparisons of the mechanistic flow boiling heat transfer models based on flow patterns including the Thome et al. three-zone heat transfer model for evaporation in microchannel and the flow pattern based model combining the Thome et al. three zone heat transfer models with the Cioncolini-Thome annular flow model for both macro- and microchannel to the database are presented. It shows that the flow pattern based model combining the three zone model with the annular flow model gives better prediction than the three zone heat transfer model alone. The flow pattern based heat transfer model favourably agrees with the experimental database collected from the literature. According to the comparison and analysis, suggestions have been given for improving the prediction methods in the future. Next, flow patterned based phenomenological models and their applications to microscale channels are presented. Finally, as an important topic, unstable and transient flow boiling phenomena in microscale channels are briefed and recommendations for future research are given. According to this comprehensive review and analysis of the current research on the fundamental issues of flow boiling, mechanisms and prediction methods in microscale channels, the future research needs have been identified and recommended. In general, systematic and accurate experimental data of flow boiling heat transfer in microscale channels are still needed although a large amount of work has been done over the past decades. The channel size effect on the flow boiling behaviours should be systematically investigated. Heat transfer mechanisms in microscale channels should be further understood and related to the corresponding flow patterns. Furthermore, effort should be made to develop and improve generalized mechanistic prediction methods and theoretical models for flow boiling heat transfer in microscale channels according to the physical phenomena/mechanisms and the corresponding flow structures. The effects of the channel size and a wide range of test conditions and fluid types should be considered in develop new methods. Furthermore, systematic experimental, analytical and modeling studies on unstable and transient flow boiling heat transfer in microscale channels should be conducted to understand the physical mechanisms and theoretical models.
A new mass transfer model has been formulated and implemented for predicting the penetration of liquid medication into an arterial wall. The model takes separate account of the transport of ...medication in the tissue and in the liquid stream. The transport equations take into consideration both diffusion and advection. An original lateral transport mechanism relating the drug concentrations in the tissue and in the advecting liquid stream is postulated and calibrated with experimental data from the literature. The special features of the mechanism are its non-Fickian nature and the accounting of the affinity (binding) of the drug to the tissue. Once established, the model was applied by means of numerical simulation to the in-artery distribution of Paclitaxel, a drug whose function is to minimize restenosis of the artery wall. The application encompassed parametric values of the duration of the drug therapy, the viscosity of the advecting fluid, and the pressure used to drive the drug into the tissue. It was found that the drug concentration at a clinically relevant depth in the artery wall varied linearly with the duration of the therapy, increased with increasing values of the driving pressure, and decreased for higher viscosities of the advecting fluid. In addition to its capabilities for predicting drug concentrations during the therapy duration period, the simulations also were able to follow the temporal variations of the drug distribution after the cessation of the therapy.
PDF