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Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2018;140(12):121501-121501-13. doi:10.1115/1.4040647.

A partial differential–integral equation has been derived to connect vapor condensation and the development of condensate film thickness in both the tangential and axial directions in a horizontal circular condenser tube. A high-order explicit numerical scheme is used to solve the strongly nonlinear equation. A simple strategy is applied to avoid possible large errors from high-order numerical differentiation when the condensate becomes stratified. A set of empirical friction factor and Nusselt number correlations covering both laminar and turbulent film condensation have been incorporated to realistically predict film thickness variation and concurrently allow for the predictions of local heat transfer coefficients. The predicted heat-transfer coefficients of film condensation for refrigerant R134a and water vapor in horizontal circular mini- and macrotubes, respectively, have been compared with the results from experiments and the results from the simulations of film condensation using computational fluid dynamics (CFD), and very good agreements have been found. Some of the predicted film condensations are well into the strong stratification regime, and the results show that, in general, the condensate is close to annular near the inlet of the condenser tube and becomes gradually stratified as the condensate travels further away from the inlet for all the simulated conditions. The results also show that the condensate in the minitubes becomes stratified much earlier than that in the macrotubes.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(12):121502-121502-7. doi:10.1115/1.4040783.

This paper quantifies the pool boiling performance of R134a, R1234yf, R513A, and R450A on a flattened, horizontal reentrant cavity surface. The study showed that the boiling performance of R134a on the Turbo-ESP exceeded that of the replacement refrigerants for heat fluxes greater than 20 kW m−2. On average, the heat flux for R1234yf and R513A was 16% and 19% less than that for R134a, respectively, for R134a heat fluxes between 20 kW m−2 and 110 kW m−2. The heat flux for R450A was on average 57% less than that of R134a for heat fluxes between 30 kW m−2 and 110 kW m−2. A model was developed to predict both single-component and multicomponent pool boiling of the test refrigerants on the Turbo-ESP surface. The model accounts for viscosity effects on bubble population and uses the Fritz equation to account for increased vapor production with increasing superheat. Both loss of available superheat and mass transfer resistance effects were modeled for the refrigerant mixtures. For most heat fluxes, the model predicted the measured superheat to within ±0.31 K.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(12):121503-121503-17. doi:10.1115/1.4041088.

Heterogeneous nucleate boiling over a flat surface has been studied through complete numerical simulations. During the growth and departure of the vapor bubble, the interface is tracked following a coupled level-set and volume of fluid approach. A microlayer evaporation model similar to Sato and Niceno [“A depletable microlayer model for nucleate pool boiling,” J. Comput. Phys. 300, 20–52 (2015)] has been deployed in this investigation. A detailed study of the changes in microlayer structure as a result of different modes of boiling scenario has been performed. The departure diameter is found to increase with an increase in substrate superheat. The predicted departure diameter has been compared with the available experimental and analytical results. A power-law curve has been obtained for depicting the growth rate of bubble depending on the degree of superheat at the wall. The space–time averaged wall-heat flux at different values of superheat temperature of substrate is obtained. Bubble growth during subcooled boiling at a low and intermediate subcooled degree has been observed through direct numerical simulations. The variations in bubble dynamics after departure in saturated and subcooled liquid states have been compared.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2018;140(12):121701-121701-10. doi:10.1115/1.4040957.

Turbulent flow heat transfer and friction penalty in triangular cross-sectional duct is studied in the present paper. The sharp corners of the duct are modified by converting it into circular shape. Five different models were designed and fabricated. Heat transfer through all the models was investigated and compared conventional triangular duct under similar conditions. The curvature radius of rounded corners for different models was kept constant (0.33 times the duct height). The numerical simulations were also performed and the obtained result validated with the experimental findings and close match observed between them. The velocity and temperature distribution is analyzed at particular location in the different models. Because of rounded corners, higher velocity is observed inside the duct (except corners) compared to conventional duct. Considerable increase in Nusselt number is seen in model-5, model-4, model-3, and model-2 by 191%, 41%, 19%, and 8% in comparison to model-1, respectively, at higher Reynolds number (i.e., 17,500). But, frictional penalty through the model-5, model-4, model-3, and model-2 increased by 287%, 54%, 18%, and 12%, respectively, in comparison to model-1 at lower Reynolds number (i.e., 3600).

Commentary by Dr. Valentin Fuster

Research Papers: Jets, Wakes, and Impingment Cooling

J. Heat Transfer. 2018;140(12):122201-122201-11. doi:10.1115/1.4041047.

This paper presents a numerical investigation of the film-cooling performance of a kind of diffusion hole with a fusiform cross section. Relative to the rectangular diffusion hole, the up- and/or downstream wall of the fusiform diffusion hole is outer convex. Under the same metering section area, six fusiform diffusion holes were divided into two groups with cross-sectional widths of W = 1.7D and W = 2.0D, respectively. Three fusiform cross section shapes in each group included only downstream wall outer convex, only upstream wall outer convex, or a combination of both. Simulations were performed in a flat plate model using a 3D steady computational fluid dynamics method under an engine-representative condition. The simulation results showed that the fusiform diffusion hole with only an outer convex upstream wall migrates the coolant laterally toward the hole centerline, and then forms or enhances a tripeak effectiveness pattern. Conversely, the fusiform diffusion hole with an outer convex downstream wall intensely expands the coolant to the hole two sides, and results in a bipeak effectiveness pattern, regardless of the upstream wall shape. Compared with the rectangular diffusion holes, the fusiform diffusion holes with only an upstream wall outer convex significantly increase the overall effectiveness at high blowing ratios. The increased magnitude is approximately 20% for the hole of W = 1.7D at M = 2.5. Besides, the fusiform diffusion holes with an outer convex upstream wall increase the discharge coefficient about 5%, within the moderate to high blowing ratio range.

Commentary by Dr. Valentin Fuster

Research Papers: Melting and Solidification

J. Heat Transfer. 2018;140(12):122301-122301-9. doi:10.1115/1.4040955.

To identify the transient and distributed internal surface heat flux of the slab mold in continuous casting process, a fuzzy inference method is proposed in this work. For temporal and spatial distribution characteristics of the internal surface heat flux of continuous casting mold, a decentralized fuzzy inference (DFI) identification scheme possessed of a decoupling characteristic in time and space is established. For each temperature measurement point, the fuzzy inference processes are, respectively, executed from the correspondingly observed temperature sequence through corresponding DFI units. In the time domain, according to sensitivity coefficients, the weighing and synthesizing processes for the decentralized inference results are performed to get the temporal compensation vector for the internal surface heat flux of mold. Then, in the space domain, according to the normal distribution function, the weighing and synthesizing processes for the temporal compensation vectors are performed to get the spatial compensation vector for the internal surface heat flux of mold. Numerical tests are carried out to research the influence of the number of thermocouples and measurement errors on the identification results, which prove the effectiveness of proposed scheme in this work.

Commentary by Dr. Valentin Fuster

Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2018;140(12):122401-122401-13. doi:10.1115/1.4040956.

An experimental study is performed to investigate water flow and heat transfer characteristics in silicon micro-pin-fin heat sinks with various pin–fin configurations and a conventional microchannel, with a length of 25 mm, a width of 2.4 mm, and a height of 0.11 mm. The micro-pin-fin heat sinks have different fin arrangements, fin shapes, and fin pitches. The results show that the micro-pin-fin heat sinks have the better overall thermal-hydraulic performance including the heat transfer enhancement and the pressure drop penalty compared to the conventional microchannel. A parametric study is carried out to investigate the effects of various pin-fin configurations on the flow and heat transfer characteristics. The linear relationship between fRe and Re is found for the water flow through the micro-pin-fin heat sinks for the first time. A new friction factor correlation is further developed based on the linear relationship between fRe and Re. Taking the effects of the various pin-fin configurations on the Nusselt number into consideration, a new Nusselt number correlation is also developed. The new correlations of friction factor and Nusselt number predict the experimental data well. An infrared thermo-imaging system was used to measure the temperature field of water heat transfer in the micro-pin-fin heat sinks and the conventional microchannel. The infrared thermo-images show the more uniform temperature profile in the transverse direction for the micro-pin-fin heat sinks than that for the conventional microchannel, which indicates the better heat transfer performance of the former than the latter. The dominant mechanism of heat transfer enhancement caused by the micro-pin-fins is the hydrodynamic effects, including fluid disturbance as well as the breakage and re-initialization of the thermal boundary layer near the wall of the heat sinks.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2018;140(12):122402-122402-15. doi:10.1115/1.4041049.

Microrib is a very promising heat transfer enhancement method for the design of scramjet regenerative cooling channels. In this paper, a three-dimensional numerical model has been built and validated to parametrically investigate the thermal behavior of transcritical n-Decane in mini cooling channels with microribs under near critical pressure. The results have shown that the height and pitch of microrib perform a nonmonotonic effect on the convective heat transfer coefficient of n-Decane inside the cooling channel and the optimal microrib parameters stay at low values due to dramatic changes of coolant thermophysical properties in the near critical region. Due to severe thermal stratification and near critical conditions, there will be a significant recirculation zone in vertical direction near microrib, and its interaction with the strong secondary flow in axial direction caused by limited channel width of mini-channel will largely enhance the local convective heat transfer and its downstream region. Besides, the dramatically changing thermophysical properties of n-Decane will lead to a locally remarkable heat transfer enhancement phenomenon similar to impingement cooling at the front edge of microribs.

Commentary by Dr. Valentin Fuster

Research Papers: Natural and Mixed Convection

J. Heat Transfer. 2018;140(12):122501-122501-9. doi:10.1115/1.4041048.

It is a common practice to use ideal thermal boundary conditions to investigate natural convection. These correspond to very good conducting walls and to very bad conducting walls. In particular, this has been the case in natural convection of viscoelastic fluids. In this paper, these conditions are generalized by taking into account the finite thermal conductivities and thicknesses of the walls in the natural convection of a viscoelastic Jeffreys fluid heated from below. The goal is to present more realistic results related to experimental conditions. The critical Rayleigh number Rc, the frequency of oscillation ωc, and the wavenumber kc have been plotted varying the properties of the walls from the case of very good thermal conductivity to very poor thermal conductivity. In order to understand the convective phenomena, two parameters are fixed and the other one varied among the nondimensional relaxation time F, the relative retardation time E, and the Prandtl number Pr of the viscoelastic fluid. The role of the relative retardation time E on the thermal instability is discussed in detail.

Commentary by Dr. Valentin Fuster

Research Papers: Thermal Systems

J. Heat Transfer. 2018;140(12):122801-122801-8. doi:10.1115/1.4041188.

The feasibility of the parameter estimation on the basis of the ensemble Kalman filter (EnKF) for a practical simulation involving model errors was investigated. The three-dimensional flow and thermal simulations for the engine compartment of a test excavator were simulated, and several unknown temperatures used for boundary conditions were estimated with the method. The estimation method was validated in two steps. First, the estimation method was tested with the influence of the model errors removed by virtually creating true values with a simulation. These results showed that the proposed parameter-estimation method can successfully estimate surface temperatures. They also suggested that the appropriate ensemble size can be evaluated from the number of unknown parameters. Second, the estimation method was tested under a practical condition including model errors by using actual measurement data. Model errors were statistically estimated using prior obtained error data concerning other design configurations, and they were added to the observation error in the EnKF. These results showed that taking model errors into account in the EnKF provides more-accurate parameter-estimation results. Moreover, the uncertainty of an estimated parameter can be evaluated with the standard deviation of its distribution.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Heat Transfer. 2018;140(12):124501-124501-5. doi:10.1115/1.4040953.

X-ray sources are used for both scientific instrumentation and inspection applications. In X-ray photoelectron spectroscopy (XPS), aluminum Kα X-rays are generated through electron beam irradiation of a copper-based X-ray anode incorporating a thin surface layer of aluminum. The maximum power operation of the X-ray anode is limited by the relatively low melting point of the aluminum. Hence, optimization of the materials and design of the X-ray anode to transfer heat away from the aluminum thin film is key to maximizing performance. Finite element analysis (FEA) has been employed to model the heat transfer of a water-cooled copper-based X-ray anode with and without the use of a chemical vapor deposited (CVD) diamond heat spreader. The modeling approach was to construct a representative baseline model, and then to vary different parameters systematically, solving for a steady-state thermal condition, and observing the effect on the maximum temperature attained. The model indicates that a CVD diamond heat spreader (with isotropic thermal properties) brazed into the copper body reduces the maximum temperature in the 4 μm aluminum layer from 613 °C to 301 °C. Introducing realistic anisotropy and inhomogeneity in the thermal conductivity (TC) of the CVD diamond has no significant effect on heat transfer if the aluminum film is on the CVD diamond growth face (with the highest TC). However, if the aluminum layer is on the CVD diamond nucleation face (with the lowest TC), the maximum temperature is 575 °C. Implications for anode design are discussed.

Commentary by Dr. Valentin Fuster

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