Enhanced Liquid Water Removal From PEM Fuel Cell Flow Channels by Superimposing Acoustic Pressure Wave on Air Flow

Enhanced water removal from the flow channel of an ex-situ PEM fuel cell test section is obtained by superimposing acoustic pressure wave on air flow prior to entering into the flow channel. Water accumulation within the flow channel was visualized with a CCD camera and liquid-gas two-phase flow pressure drop was measured along the flow channel. Acoustic pressure waves were superimposed in sine waves at different frequencies between 20 and 120 Hz with a 20-Hz interval. Results indicated that water accumulation in the flow channel was lowest when acoustic pressure waves were superimposed at 80 Hz on air flow. For experiments with no acoustic vibration, the average water slug cumulative area for three runs was obtained at 288.6 mm2 while this average was as low as 43.9 mm2 for experiments conducted at 80 Hz. For other frequencies tested (20, 40, 60, 100, and 120 Hz), water accumulation within the flow channel was less than that for experiments with no vibration but the accumulation of water was still greater than experiments conducted at 80 Hz. The two-phase flow pressure drops were also lowest for experiments conducted at 80 Hz while the highest pressure drops were obtained in experiments with no acoustic vibration. Droplets were also visualized from a side-view angle in a goniometer in order to obtain contact angles. Images showed droplet oscillation under the influence of acoustic vibration. For the three superficial air velocities tested in this study (1.30, 1.82, and 2.30 m/s) the contact angle hysteresis were almost identical with an average value around 40°.

Optimization of Hybrid Hydrophilic-Hydrophobic Surfaces for Dropwise Condensation Enhancement

A langrangian-based phenomenological model, following the evolution of the individual droplets nucleating at random positions, is applied to the simulation and optimization of hybrid, mixed hydrophobic and hydrophilic surfaces. The proposed mathematical model was originally developed to simulate droplets pattern evolution in the framework of in-flight icing problems and takes into account the surface tension effects (via the advancing and receding contact angle values), the external force balance on the single droplets, as well as the both the condensing and coalescence process. Here, the model is extended to the case of an arbitrary curved substrate surface and a parametric analysis of different cases is carried out, looking for the parameters that help to identify the optimal design for a given set of wettability properties, nucleation site density and wet surface condensation rates. A discussion on the sensitivity of the solutions with respect to the expected high uncertainties on the estimate of some of these parameters in actual practical problems is also presented, in order to assess the effectiveness of the simulation as a design tool. The analysis is carried out for condensation enhancement on both plane surfaces and mini or micro tubes. Comparison with experimental, open literature data ensure the reliability of the approach for both geometries.

Flow Maldistribution Effects on the Temperature Uniformity in Double-Layered Microchannel Heat Sinks

A numerical investigation is carried out on the effects of flow maldistribution on the temperature uniformity and overall thermal resistance in double-layered microchannel heat sinks. Different flow maldistribution models accounting for the effects of some typical header designs are considered together with different combinations of the average inlet velocity in the two layers of microchannels for a given total mass flow rate. The numerical simulations are carried out using an in-house FEM procedure previously developed by the authors for the analysis of cross-flow microchannel heat exchangers.

Performance of Concentrator Photovoltaic Systems Integrated With Double Layer Microchannel Heat Sink

In this study, a new design of double layer microchannel heat sink (DL-MCHS) has been monolithically fabricated using 3D metal printer and experimentally examined as a heat sink for concentrator photovoltaic (CPV) systems. Single phase liquid cooling using ethanol and flow boiling cooling using NOVEC-7000 coolant in the designed DL-MCHS are experimentally compared. The results proved that using the flow boiling cooling technique for the CPV systems attained a lower solar cell temperature with high temperature uniformity. In more details, flow boiling in counterflow (CF) operated DL-MCHS, attained a very low solar cell temperature close to the NOVEC-7000 boiling point with temperature uniformity of 0.2 °C over a wide range of coolant flow rate from 25–250 ml/hr.

Thermoreflectance Wall Temperature Measurement in Annular Two-Phase Flow

A thermoreflectance method to measure wall temperature in two-phase annular flow is described. In high heat flux conditions, momentary dry-out occurs as the liquid film vaporizes, resulting in dramatic decreases in heat transfer coefficient. Simultaneous liquid and vapor thermoreflectance measurements allow calculations of instantaneous and time-averaged heat transfer coefficients. Validation, calibration and uncertainty of the technique are discussed.

Review on Bubble Dynamics in Microchannel Heat Sink

Inception of the boiling, in pool or flow boiling, is the formation of the vapour bubble at active nucleation site. The bubble dynamics plays an important role in the boiling process. It is critical as it unfolds many facets especially when channel size is reduced to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behaviour in microchannel. In this paper, the bubble dynamics which include bubble nucleation at nucleation site, its growth, departure and motion along the flow in a microchannel are discussed in details. Different models are developed for the critical cavity radius are compiled and observed that they show large variation when compare. The bubble growth models are compiled and concluded that a development of more generalized bubble growth model is necessary to account for the inertia controlled and thermal diffusion controlled regions. The bubble at the nucleation site in a microchannel grows under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric variations of these forces are critically studied and reckoned that the slope of these forces seems to be reduced beyond 500 μm. Eventually, possible impact of the various factors such as operating conditions, geometrical parameters, and thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

Parametric Study of Vortex Generator Effects in an Additive Manufactured Minichannel Heat Exchanger

Heat exchangers with mini- and micro-channel components are capable of high energy exchange due to their incumbent large surface area to volume ratio. Concurrently, recent advances in additive manufacturing simplify the creation of metallic minichannels that incorporate turbulators for heat transfer enhancement. As part of the development of a minichannel heat exchanger with turbulators, this study analyzes the three-dimensional conjugate heat transfer and laminar flow in a minichannel heat exchanger equipped with rectangular winglet vortex generators (VGs) through numerical simulation. The minichannels have a hydraulic diameter of 2.86 mm and are assumed to be made from aluminum alloy AlSi10Mg. This material is one of the popular alloys in the additive manufacturing industry (three-dimensional (3D) printing) because of its light weight and beneficial mechanical and thermal properties. The working fluid is distilled water with temperature-dependent thermal properties. The minichannel is heated by a constant heat flux of 5 W cm−2 and the Reynolds number is varied from 230 to 950. The simulations are performed using the COMSOL® platform, which solves the governing mass, momentum, and energy equations based on the finite element method. The effect of the VG design parameters, which include VG angle of attack, height, length, thickness, longitudinal pitch, and distance from the sidewalls, is investigated. It is found that the generation of three-dimensional vortices caused by the presence of the vortex generators can notably boost the convective heat transfer, at the cost of increased pressure drop, potentially reducing the heat exchanger size for a given heat duty. A sensitivity analysis indicates that the angle of attack, VG height, VG length, and longitudinal pitch have the most significant effects on the heat transfer and flow friction characteristics. In contrast, the VG thickness and distance from the sidewalls only had minor influences on the heat exchanger performance over the studied range of design parameters.

Numerical Simulation of Liquid-Liquid Taylor Flow With Heat Transfer

Numerical simulation of Taylor flows presents several challenges. At the dynamic interface physical properties are discontinuous, which is especially challenging for the thin film between the droplet and the wall. Phase-field methods, which are derived from thermodynamic principles, define the interface as a smooth transition between phases. By coupling the Cahn-Hilliard equation with the Navier-Stokes and energy equation, both interface dynamics and heat transfer can be captured. In the work presented, the resulting system of equations are solved by a parallel h-adaptive least-squares spectral element method. To approximate the solution with sufficient numerical accuracy, C1 Hermite basis functions and a space-time formulation have been applied. It is widely accepted in the literature that the droplet characteristics such as length, velocity and dynamic interaction among them affect the heat transfer properties of Taylor flow. To gain understanding, their effect on heat transfer and pressure drop for liquid-liquid Taylor flow in microchannels must be studied in more detail.

Performance Analysis of Concentrator Photovoltaic/Microchannel Heat Sink System Using Nanofluid

In this study, the performance of concentrator photovoltaic (CPV) cell enhanced by using double layer microchannel heat sink (DL-MCHS) with nanofluid is investigated. Pure ethanol and 0.2 % Vol. Al2O3-ethanol are utilized to reduce the solar cell temperature under indoor solar concentration ratio of 5.7 Suns. The designed DL-MCHS is monolithically fabricated from Maraging steel using 3D metal printer. The experimental results showed that using parallel flow (PF) operation mode of the designed DL-MCHS is favourable for cooling the CPV system compared with the counter flow (CF) operation mode. In the cooled CPV using PF mode, the open circuit voltage enhancement is about 12.7% in comparison to the uncooled case. The nanofluid results also showed a reduction in the solar cell temperature in comparison with the pure coolant. The current results can be used as a validation step for accurate numerical modelling of nanofluid applications in CPV system cooling.

Numerical Study on Gas-Yield Power-Law Fluid in T-Junction Minichannel

In this study, a computational examination of Taylor bubbles was performed for gas/non-Newtonian fluid two-phase flows developed in a minichannel T-junction mixer with a hydraulic diameter of 1 mm. The investigations employed three separate aqueous xanthan gum solutions at concentrations of 0.05, 0.1 and 0.15 w/w, which are referred to as non-Newtonian (yield power-law) fluids. The effective concentration of the xanthan gum solutions and superficial velocity of the inlet liquid phase on the length, velocity, and shape of the Taylor bubbles was studied using the ANSYS FLUENT 19 software package. The simulation results show an increase in bubble velocity with increasing film thickness, particularly in solutions of higher viscosity XG-0.15%. Furthermore, bubble lengths decreased as the xanthan gum concentrations increased, but bubble shapes underwent alterations when the concentrations increased. Another interesting result of the tests shows that when the liquid inlet velocity increases, bubble lengths decrease during lower liquid superficial velocity, whereas during higher velocities, they change only slightly after increases in concentration. Finally, with increasing XG concentration, the liquid film thickness around the bubble increased. The results show good agreement with correlations after modifying a capillary number (Ca*) for non-Newtonian liquids in all cases.