Single Liquid Flow Research Articles

Evaluating the effects of cavitating refrigerant flow on noise radiation from electronic expansion valve: a numerical study

The electronic expansion valve (EEV) is a critical component in air conditioners, responsible for regulating refrigerant flow rate and controlling the cooling or heating performance. However, it is also a significant source of noise. When the valve opening ratio is low, the increased flow velocity of the refrigerant can cause pressure to drop below the vapor pressure and result in a cavitation flow. This type of flow is known to produce higher levels of flow noise compared to single-phase liquid refrigerant flow. A Large Eddy Simulation is utilized to compute the internal flow of the EEV, incorporating a homogeneous mixture model to model cavitation phenomena. The structural vibration of the EEV and its inlet/outlet pipe wall is computed using high-resolution finite element methods with excitation force from internal wall pressure fluctuations. The near and far field noise radiation from the structure is calculated using finite and boundary element methods, respectively. The developed numerical methods are applied to two operating conditions: single-phase and two-phase. The results indicate that the noise radiation from the EEV is 6.5 dB higher in the cavitating flow than in the single liquid flow, closely matching the measured data. The current numerical methodology is expected to be useful in designing low-noise EEV units.

Thermal Performance Analysis of Slug Flow in Square Microchannels

Thermal performance enhancement of microchannel heat sinks for electronics cooling is becoming more and more necessary. To this end, microchannels with noncircular cross-sections conveying immiscible droplets have been employed in this study and geometric manipulations are applied to enhance fluid mixing and consequently achieve a higher rate of heat removal. Three-dimensional numerical simulations are performed using volume of fluid method for channels of 100 µm hydraulic diameter. Constant wall temperature is chosen as the boundary condition for heating section of the channels. Effects of parameters such as adding curvature to the side walls and varying the entering velocity of the base liquid on heat transfer rate are studied. A performance coefficient is used to evaluate the relative impact of increase in both the Nusselt number and pressure drop as a result of adding curvature to the channel walls. Results of the study showed that slug flow in curved channels is capable of improving the thermal performance in comparison with single liquid flow in straight channels and in best case, can improve the performance up to 50%.

Investigation of permeability, formation factor, and porosity relationships for Mesaverde tight gas sandstones using random network models

Replicating the pore topology/structure of tight gas reservoirs is essential to model fluid flow through such porous media. Constitutive relationships between the macroscopic properties of the medium can often help with such modeling efforts. Permeability and formation factor are rock properties providing useful information for assessing the potential of hydrocarbon recovery. Pore topology/structure and pore–throat radius distributions are the major factors having influence on permeability and formation factor estimation. A stochastic random generation algorithm is employed to study the effect of pore structure and geometries on the relationships of formation factor–permeability and permeability–porosity on physically realistic 3D random networks. These relationships are derived by constructing two sets of physically equivalent pore networks of tight porous media and are validated using experimental measurements of Mesaverde tight gas sandstones. The first set of networks were based on Berea sand network properties, which are then reduced and derived using a Weibull truncated equation to produce physically sound tight porous media. The second equivalent networks are constructed according to experimentally derived throat size distributions obtained from ambient mercury injection capillary pressure for 17 selected core samples from three Mesaverde tight gas sandstones basins in the U.S. Imperial college Pore-Scale Modeling software is used to model the single liquid flow properties through constructed equivalent networks. The estimated porosity, absolute liquid permeability and formation factor of the constructed physically equivalent networks are in good agreement with measured data obtained by Byrnes et al. (Analysis of critical permeability, capillary and electrical properties for Mesaverde tight gas sandstones from Western US basins: final scientific. Technical report submitted to DOE and NETL 355, 2009). However, a variation between estimated absolute permeability to liquid and measured routine gas permeability is accounted in core samples that have measured permeability smaller than 0.1 mD. Networks based on Berea sand properties show qualitative agreement between modeled data points and experiment data. However, modeled data are off by two orders of magnitude and all fall more or less on the same line.

Numerical Simulation of Seed Bubble Thermal Control Microchannel Boiling Heat Transfer under High Heat Flux

Since the mechanism of seed bubble triggering boiling flow heat transfer in microchannels could not be understood comprehensively,the volume of fluid(VOF) method is used to simulate the seed bubble triggering boiling flow in a circular microchannel under the high heat flux for four triggering frequencies.An accuracy and simple vapor-liquid phase change model is proposed to treat boiling flow process properly.Simulation results show that triggering frequency is one of the key parameters to control system heat transfer and stability.At low frequency(≤100 Hz),seed bubbles contribute least effects,and wall superheats vary with apparent millisecond-level fluctuation amplitudes.At high frequency(1 000 Hz or higher),with the frequency increasing,the fluctuation amplitude of wall temperatures can be successfully suppressed,the heating wall superheats attains the minimum degree.But triggering frequency can approach the saturation value(f = 2 000 Hz).In addition,the transient flow patterns and local heat transfer coefficient during the stable stage for saturated frequency are studied,continuous phase change heat transfer can be achieved,and the local heat transfer coefficient in the elongated bubble regimes can sustain 2-3 times the single liquid flow heat transfer.

Large Convective Heat Transfer Enhancement in Microchannels With a Train of Coflowing Immiscible or Colloidal Droplets

We show that heat transfer in microchannels can be considerably augmented by introducing droplets or slugs of an immiscible liquid into the main fluid flow. We numerically investigate the influence of differently shaped colloidal or simply pure immiscible droplets to the main liquid flow on the thermal transport in microchannels. Results of parametric studies on the influence of all major factors connected to microchannel heat transfer are presented. The effect of induced Marangoni flow at the liquid interfaces is also taken into account and quantified. The calculation of the multiphase, multispecies flow problem is performed, applying a front tracking method, extended to account for nanoparticle transport in the suspended phase when relevant. This study reveals that the use of a second suspended liquid (with or without nanoparticles) is an efficient way to significantly increase the thermal performance without unacceptably large pressure losses. In the case of slug-train coflow, the Nusselt number can be increased by as much as 400% compared with single liquid flow.

Transport of bubbles in square microchannels

Liquid/gas flows are experimentally investigated in 200 and 525 μm square microchannels made of glass and silicon. Liquid and gas are mixed in a cross-shaped section in a way to produce steady and homogeneous flows of monodisperse bubbles. Two-phase flow map and transition lines between flow regimes are examined. Bubble velocity and slip ratio between liquid and gas are measured. Flow patterns and their characteristics are discussed. Local and global dry out of the channel walls by moving bubbles in square capillaries are investigated as a function of the flow characteristics for partially wetting channels. Two-phase flow pressure drop is measured and compared to single liquid flow pressure drop. Taking into account the homogeneous liquid fraction along the channel, an expression for the two-phase hydraulic resistance is experimentally developed over the range of liquid and gas flow rates investigated.

The cooling effect of liquid flow on the focussed ultrasound-induced heating in a simulated foetal brain.

There is a need to investigate the thermal effects of diagnostic ultrasound (US) to assist the development of appropriate safety guidelines for obstetric use. The cooling effect of a single liquid flow channel was measured in a model of human foetal brain and skull bone heated by a focussed beam of simulated pulsed spectral Doppler US. Insonation conditions were 5.7 micros pulses, repeated at 8 kHz from a focussed transducer operating with a centre frequency of 3.5 MHz, producing a beam of -6 dB diameter of 3.1 mm at the focus and power outputs of up to 255 +/- 5 mW. Brain perfusion was simulated by allowing distilled water to flow at various rates in a 2 mm diameter wall-less channel in the brain soft tissue phantom material. This study established that the cooling effect of the flowing water; 1. was independent of the acoustic source power, 2. was more effective close to the flow channel, for example, there was a marked cooling at a distance of 1 mm and negligible cooling at a distance of 3 mm from the channel; and 3. initially increased at low flow rates, but further increase above normal perfusion had very little effect.

Particle-to-liquid mass transfer in gas-liquid-solid fluidization.

Mass transfer from solid particles to liquid in gas-liquid-solid fluidization was experimentally investigated by means of an electrochemical method with 7-9 particle electrodes fixed at various radial positions in a column of 82.9 mm i.d. Supplementary measurements were also conducted for the solid-to-liquid mass transfer in single liquid flow, gas-liquid two-phase upflow and liquid-solid fluidization. The effects of particle size and liquid and gas flow rates on the solid-to-liquid mass transfer coefficient, Ks, in all the systems were investigated and the mass transfer coefficient was well correlated by the following equation proposed by Ohashi et al. for Ks in a packed bed with single liquid flow. Shs = 2 + O.51(E1/3dp4/3/V1)0.06Sc1/3where E is the energy dissipation rate per unit mass of liquid, dp the particle diameter, vl the kinematic viscosity of liquid, Shs the Sherwood number and Sc the Schmidt number. The value of E was expressed by the sum of the kinetic energy supply terms due to viscous drag force across a particle and to gas agitation.

Wall-to-liquid mass transfer in packed and fluidized beds with gas-liquid concurrent upflow.

The wall-to-liquid mass transfer coefficient, kw, was measured for both surfaces of a coaxially immersed tube and a column wall in packed and fluidi/ed bed systems with gas-liquid concurrent upflow. Supplementary measurements of kw were carried out in openpipe liquid flow, gas-liquid two-phase upflow and packed and fluidized beds with single liquid flow. The value of kw in the three-phase fluidized bed increased with increasing gas flow rate, deviating from the value in the liquid-solid fluidized bed and approaching the value in the gas-liquid two-phase upflow, while it passed through a maximum value with respect to the liquid flow rate. The value of kw in the packed-bed operation increased with increasing liquid flow rate and with increasing gas flow rate. The values of kw for the inner tube and the column wall were shown to agree with each other. The values of kw in all the gas-liquid, liquid-solid and gas-liquid-solid systems examined were correlated well by a unified equation in terms of the energy dissipation rate per unit mass of liquid. An analogy existed between wall heat transfer and wall mass transfer in all the multiphase flow systems examined.