Two-phase flow instabilities in a silicon microchannels heat sink

Two-phase flow instabilities are highly undesirable in microchannels-based heat sinks as they can lead to temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations. This work is an experimental study of boiling instabilities in a microchannel silicon heat sink with 40 parallel rectangular microchannels, having a length of 15 mm and a hydraulic diameter of 194 μm. A series of experiments have been carried out to investigate pressure and temperature oscillations during the flow boiling instabilities under uniform heating, using water as a cooling liquid. Thin nickel film thermometers, integrated on the back side of a heat sink with microchannels, were used in order to obtain a better insight related to temperature fluctuations caused by two-phase flow instabilities. Flow regime maps are presented for two inlet water temperatures, showing stable and unstable flow regimes. It was observed that boiling leads to asymmetrical flow distribution within microchannels that result in high temperature non-uniformity and the simultaneously existence of different flow regimes along the transverse direction. Two types of two-phase flow instabilities with appreciable pressure and temperature fluctuations were observed, that depended on the heat to mass flux ratio and inlet water temperature. These were high amplitude/low frequency and low amplitude/high frequency instabilities. High speed camera imaging, performed simultaneously with pressure and temperature measurements, showed that inlet/outlet pressure and the temperature fluctuations existed due to alternation between liquid/two-phase/vapour flows. It was also determined that the inlet water subcooling condition affects the magnitudes of the temperature oscillations in two-phase flow instabilities and flow distribution within the microchannels.     

Two-phase flow distribution of air–water annular flow in a parallel flow heat exchanger

The air and water flow distribution are experimentally studied for a round header – flat tube geometry simulating a parallel flow heat exchanger. The number of branch flat tube is 30. The effects of tube outlet direction, tube protrusion depth as well as mass flux, and quality are investigated. The flow at the header inlet is identified as annular. For the downward flow configuration, the water flow distribution is significantly affected by the tube protrusion depth. For flush-mounted configuration, most of the water flows through frontal part of the header. As the protrusion depth increases, more water is forced to the rear part of the header. The effect of mass flux or quality is qualitatively the same as that of the protrusion depth. Increase of the mass flux or quality forces the water to rear part of the header. For the upward flow configuration, however, most of the water flows through rear part of the header. The protrusion depth, mass flux, or quality does not significantly alter the flow pattern. Possible explanations are provided based on the flow visualization results. Negligible difference on the water flow distribution was observed between the parallel and the reverse flow configuration.     

A semi-empirical model of two-phase flow of refrigerant-134a through short tube orifices

Measurements were conducted on Refrigerant-134a flowing through short tube orifices with length-to-diameter (L/D) ratios ranging from 5 to 20. Both two-phase and subcooled liquid flow conditions entering the short tube were examined for upstream pressures ranging from 896 to 1448 kPa and for qualities as high as 10% and subcoolings as high as 13.9°C. Data were analyzed as a function of the main operating variables and tube geometry. Semi-empirical models for both single- and two-phase flow at the inlet of the short tubes were developed to predict the mass flow of Refrigerant-134a through short tube orifices.

Choked flow conditions for Refrigerant-134a were typically established when downstream pressures were reduced below the saturation pressure corresponding to the inlet temperature. The flow rate strongly depended on the upstream pressure and upstream subcooling/quality. The mass flow also depended on cross-sectional area and short tube length. The mass flow model utilized a modified orifice equation that formulated the mass flow as a function of normalized operating variables and short tube geometry. For a two-phase flow entering the short tube, the modified orifice equation was corrected using a theoretically derived expression that related the liquid portion of the mass flow under two-phase conditions to a flow that would occur if the flow were a single-phase liquid. It was found that for sharp-edged short tubes with single- and two-phase flow, approximately 95% of the measured data and model's prediction were within ±15% of each other.     

Multi-channel effect of condensation flow in a micro triple-channel condenser

Multi-channel effect is important to understand transport phenomenon in phase change systems with parallel channels. In this paper, visualization studies were performed to study the multi-channel effect in a silicon triple-channel condenser with an aspect ratio of 0.04. Saturated water vapor was pumped into the microcondenser, which was horizontally positioned. The condenser was cooled by the air natural convention heat transfer in the air environment. Flow patterns are either the annular flow at high inlet vapor pressures, or a quasi-stable elongated bubble at the microchannel upstream followed by a detaching or detached miniature bubble at smaller inlet vapor pressures. The downstream miniature bubble was detached from the elongated bubble tip induced by the maximum Weber number there.     

EXPERIMENTAL FLOW STUDY WITHIN A SELF OSCILLATING COLLAPSIBLE TUBE

The flow field within a self-excited flexible tube was studied by employing flow visualization, velocity and pressure measurements. Under low positive tansmural pressures at the tube inlet (of the order of 50 mm H₂O) the tube was set to an oscillatory motion, the initiation of which was due to a flow asymmetry. Namely, although initially the flow was separated from both sides of the formed divergent nozzle close to the tube outlet, at an arbitrary instant this became attached to one side, but stayed separated in the remaining part of the nozzle. When this flow asymmetry occurred, the walls approached each other and the tube neck formed there by started oscillating streamwise, setting the tube to an oscillatory motion. During the downstream motion of the tube neck, it was shrunk, causing a twofold increase of the local velocities compared to the inlet ones, which remained almost constant. On the contrary, when moving upstream, the tube neck expanded, causing a flow reversal in this area and a flow deceleration in the remaining part of the tube. The pressure signals upstream and downstream of the tube exhibited a phase difference, the latter leading, taking an order of magnitude higher values than the first one.     

Computational study of aeration for wastewater treatment via ventilated pump-turbine

Large eddy simulations were performed on a modular pump-turbine to study oxygen dissolution inside the draft tube. Air injection was applied over the runner cone surface during turbine operation. Data regarding bubble size, void fraction and interfacial area concentration were presented to understand their influence on oxygen dissolution. Transient single phase and multiphase flow simulations were carried out to investigate the influence of air injection and dissolution within the flow field and turbine performance. Multiphase simulations were conducted by using the mixture multiphase model. The mathematical modeling of oxygen dissolution employed was validated by comparing predicted oxygen dissolution against experimental measurements performed by Zhou et al. (2013). The averaged dissolved oxygen concentration in the range of 1.2–1.4 mg/l was obtained; which is sufficient for an active aerobic microorganism activity for wastewater treatment processes. Dissolution efficiency and the amount of averaged dissolved oxygen inside the draft tube were sensitive to the inlet bubble size. The efficiency of the dissolution increases strongly as the inlet bubble size was reduced. The obtained results revealed that vortex suppression was achieved through air admission within multiphase flow simulation. Moreover, the power generation of the turbine was hardly influenced by the aeration through the runner cone.     

The effect of internal surface modification on flow instabilities in forced convection boiling in a horizontal tube

This paper presents the experimental result of a study on the effects of heat transfer enhancement on two-phase flow instabilities in a horizontal in-tube flow boiling system. Five different heat transfer surface configurations and five different inlet temperatures are used to observe the effect of heat transfer enhancement and inlet subcooling. All experiments are carried out at constant heat input, system pressure and exit restriction. Dynamic instabilities, namely pressure-drop type, density-wave type and thermal oscillations are found to occur for all the investigated temperatures and enhancement configurations, and the boundaries for the appearance of these oscillations are found. The effect of the enhancement configurations on the characteristics of the boiling flow dynamic instabilities is studied in detail. The comparison between the bare tube and the enhanced tube configurations are made on the basis of boiling flow instabilities. Differences among the enhanced configurations are also determined to observe which of them is the most stable and unstable one. The amplitudes and periods of pressure-drop type oscillations and density-wave type oscillations for tubes with enhanced surfaces are found to be higher than those of the bare tube. The bare tube is found to be the most stable configuration, while tube with internal springs having bigger pitch is found to be the most unstable one among the tested tubes. It is found that system stability increases with decreasing equivalent diameter for the same type heater tube configurations; however, on the basis of effective diameter there is no single result such as stability increase/decrease with increasing/decreasing effective diameter.     

Liquid flow patterns about barbotage bubbles

This paper summarizes the results of a flow visualization study on the liquid motion around barbotage bubbles during growth and departure. Flow patterns, as well as for the first time, instantaneous velocities, are reported as a function of time and location about the bubbles. The experiments, employing the hydrogen-bubble technique and high-speed cine photography, were with: water as the liquid, air as the bubbled gas, orifice diameters of 0.116 and 0.252 cm, and different air flow rates; the two limiting cases of constant supply pressure and constant volumetric flow rate were covered. It was found that the liquid around a barbotage bubble assumes two velocity maxima, the first an outward maximum during bubble growth and the second in the opposite direction approximately at the time of bubble departure; further, liquid velocities were found to be higher close to the bubbling site. Certain differences in liquid velocities between the constant pressure and constant flow cases are explained in terms of available theoretical solutions to the bubble growth rate. Qualitative comparisons of the barbotage liquid flow patterns and those recently reported for boiling flow patterns are also presented.     

Refrigerant distribution in a parallel flow heat exchanger having vertical headers and heated horizontal tubes

Refrigerant R-410a flow distribution is experimentally studied in a test section simulating a parallel flow heat exchanger having vertical headers with two pass configuration. Tubes are heated to yield a test section outlet superheat of 5 °C with inlet quality of 0.3. Mass flux is varied from 50 kg/m² s to 70 kg/m² s. Effects of inlet and outlet locations are investigated in a search for an optimum configuration. Results show that, significant liquid flows through bottom channels, and less liquid is supplied to top channels. As for the inlet location, better flow distribution (pressure drop as well) is obtained for top inlet as compared with middle inlet. As for the outlet location, top or bottom outlet is better than middle outlet. Correlations are developed for the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream. The correlations may be used to predict the liquid or gas distribution in a parallel flow heat exchanger having vertical headers. A novel thermal performance evaluation method, which accounts for tube-side flow mal-distribution is proposed.     

Critical heat flux of liquid helium (I) in forced convective boiling

For liquid helium (I) flowing through a uniformly heated vertical tube of 1 mm i.d., critical heat flux (CHF) of forced convective boiling has been measured under the conditions of a pressure of 0.199 MPa with vapor-to-liquid density ratio, of 0.409, length-to-diameter ratios from 25 to 200, mass velocities from 11 to 108 kg m^?2 s^?1, and inlet subcooling enthalpy from ?3.5 to +7.0 kJ kg^?1. The experimental results thus obtained are analyzed revealing that the high pressure character, which has already been observed for water and Freons at extremely high pressures, is certainly found at high mass velocities for liquid helium too, though its appearance is restricted within very narrow limits due to the phenomenon that the wall temperature situation identifying the CHF condition fades away when the mass velocity becomes high.     

Transient critical heat flux in loss-of-flow-accidents (L.O.F.A.)

Loss of flow transients with reference to L.O.F. accidents in nuclear reactor cores have been systematically studied employing freon 12 as coolant. Two pressures (with reference to BWR and PWR characteristic liquid to vapour densities ratios), three periods of the coast-down flow transients during the simulated pump trips, and different specific mass flow rates have been investigated. The uniformly heated channel (L = 200 cm, D = 0.75 cm), instrumented with wall thermocouples and inlet to outlet differential pressure enabled recording of the following transients, inlet specific mass flowrate, inlet pressure, inlet to outlet Δp, inlet fluid temperature, outlet wall temperature, outlet bulk temperature.Through the wall temperature being close to the outlet it is possible to detect the onset of DNB and hence the time to DNB from the beginning of the flow transient. All the experimental runs (105) have been systematically compared with the G.E. (PEPE) code with the introduction of a CNEN DNB freon correlation. The results enable a series of conclusions which are extensively shown in the paper.     

Equal quality distribution of gas–liquid two-phase flow by partial separation method

This paper proposes a new method for equal quality distribution of gas–liquid two-phase flow by partial separate-phase distribution with a dual-header distributor. The upper and liquid (lower) headers are interconnected with five vertical downward arms. A gas–liquid two-phase mixture enters the distributor from the upper header where most of the liquid of the mixture is removed through the downward arms into the liquid header. Hence, firstly, the remaining gas-rich fluid can be uniformly distributed into the outlet branches, and then secondly, the liquid collected in the liquid header can be uniformly re-distributed into the individual outlet branches. Because both distribution processes are conducted in the condition of single or near single-phase flow, mal-distribution of the two-phase flow is essentially eliminated, and a satisfactory equal quality distribution of gas–liquid two-phase flow is reached. Experiments were conducted in an air–water two-phase flow test loop. The inner diameter of the inlet pipe was 60 mm, the superficial velocity ranges of gas and liquid were 3–32 m/s and 0.02–0.17 m/s respectively, and the quality ranged from 0.02 to 0.44. The flow pattern in the inlet pipe included stratified flow, wavy stratified, slug flow, and annular flow. The experimental results showed that this new method could significantly improve the distribution performance of the two-phase flow. The maximum quality deviation between each outlet branch and the inlet pipe is less than ±1% under the conditions of stratified, wavy stratified and slug flows in the upper header, and less than ±5% in annular flow.     

Flow patterns and boundary conditions for inlet and outlet conduits of large pump system with low head

The flow patterns in the inlet and outlet conduits have a decisive effect on the safe, stable, and highly efficient operation of the pump in a large pumping station with low head. The numerical simulation of three-dimensional （3D） turbulence flow in conduits is an important method to study the hydraulic performance and conduct an optimum hydraulic design for the conduits. With the analyses of the flow patterns in the inlet and outlet conduits, the boundary conditions of the numerical simulation for them can be determined. The main obtained conclusions are as follows： （i） Under normal operation conditions, there is essentially no pre-swirl flow at the impeller chamber inlet of an axial-flow pump system, based on which the boundary condition at the inlet conduit may be defined. （ii） The circulation at the guide vane outlet of an axial-flow pump system has a great effect on the hydraulic performance of the outlet conduit, and there is optimum circulation for the performance. Therefore, it is strongly suggested to design the guide vane according to the optimum circulation. （iii） The residual circulation at the guide vane outlet needs to be considered for the inlet boundary condition of the outlet conduit, and the value of the circulation may be measured in a specially designed test model.     

Heat transfer,pressure drop and flow patterns during flow boiling of R407C in a horizontal microfin tube

The heat transfer, pressure drop and flow patterns during flow boiling of R407C in a horizontal microfin tube have been investigated. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long. It is heated electrically. The experiments have been performed at saturation temperatures between −30°C and +10°C. The mass flux was varied between 25 and 300 kg/m²/s, the heat flux from 20,000 W/m² down to 1,000 W/m². The vapour quality was kept constant at 0.1, 0.3, 0.5, 0.7 at the inlet and 0.8, 1.0 at the outlet, respectively. The measured heat transfer coefficient is compared with the correlations of Cavallini et al., Shah as well as Zhang et al. Cavallini’s correlation contains seven experimental constants. After fitting these constants to our measured values, the correlation achieves good agreement. The measured pressure drop is compared to the correlations of Pierre, Kuo and Wang as well as Müller-Steinhagen and Heck. The best agreement is achieved with the correlation of Kuo and Wang. Almost all values are calculated within an accuracy of ±30%. The flow regimes were observed. It is shown, that changes in the flow regime affect the heat transfer coefficient significantly.     

Effect of the conical-shape on the performance of vortex tube

The present study focuses on the effect of conical shape in the cold side of the Ranque-Hilsch vortex tube which is shown to have a considerable influence on the system performance. A vortex tube is a simple circular tube with no moving parts which is capable to divide a high pressure flow into two relatively lower pressure flows with temperatures higher and lower than the incoming flow. A three-dimensional computational fluid dynamic model is used to analyse the mechanisms of flow inside a vortex tube. The SST turbulence model is used to predict the turbulent flow behaviour inside the vortex tube. The geometry of a vortex tube with circumferential inlet slots as well as axial cold and hot outlet is considered. Performance curves temperature separation versus cold outlet mass fraction are calculated for a given inlet mass flow rate and varying outlet mass flow rates.     

Transition of plug to slug flow and associated fluid dynamics

Transition of plug to slug flow is associated with bubble detachment from elongated bubble tail or bubble entrainment inside the liquid slug. The mechanism responsible for this transition was earlier identified by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) based on the shape of the hydraulic jump observed at elongated bubble tail region. The transition mechanism reported by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) was only based on their flow visualization study. Plug to slug transition and associated dynamics of bubble detachment from the elongated bubble is analysed in the present paper using flow visualization and local velocity measurements. Experiments are reported for 13 different inlet flow conditions of air and water phases. Images of plug/slug flow structures are captured at a rate of 4000 FPS using FASTCAM Photron camera and the local values of axial liquid velocity are measured using LDV system synchronised with a 3D automated traverse system. LDV measurement of local liquid velocity in the liquid slug and liquid film establishes the reason for detachment of bubbles from the slug bubble tail.     

Phase split of nitrogen/non-Newtonian fluid two-phase flow at a micro-T-junction

An experimental investigation has been undertaken to understand the phase split of nitrogen gas/non-Newtonian liquid two-phase flow passing through a 0.5 mm T-junction that oriented horizontally. Four different liquids, including water and aqueous solutions of carboxymethyl cellulose (CMC) with different mass concentrations of 0.1, 0.2 and 0.3 wt%, were employed. Rheology experiments showed that different from water, CMC solutions in this study are pseudoplastic non-Newtonian fluid whose viscosity decreases with increasing the shear rate. The inlet flow patterns were observed to be slug flow, slug–annular flow and annular flow. The fraction of liquid taken off at the side arm for nitrogen gas/non-Newtonian liquid systems is found to be higher than that for nitrogen gas/Newtonian liquid systems in all inlet flow patterns. In addition, with increasing the pseudoplasticity of the liquid phase, the side arm liquid taken off increases, but the increasing degree varies with each flow pattern. For annular flow, the increasing degree is much greater than those for slug and slug–annular flows.     

Heat and mass transfer characteristics of air bubbles and hot water by direct contact

 This paper has dealt with direct contact heat and mass transfer characteristics of air bubbles in a hot water layer. The experiments were carried out by bubbling air in the hot water layer under some experimental conditions of air flow rate, inlet air temperature and humidity as a dispersion fluid, and hot water temperature and hot water layer depth as a continuous fluid. Heat transfer and evaporation of water vapor from hot water to air bubbles occurred during air bubbles ascending into the hot water. Air bubble flow patterns were classified into three regions of independent air bubble flow, transition and air bubble combination growth. Non-dimensional correlation equations of direct contact heat and mass transfer between air bubbles and hot water were derived by some non- dimensional parameters for three regions of bubble flow pattern. Received on 14 July 2000 / Published online: 29 November 2001     

Explosive boiling of water in parallel micro-channels

The objective of this study is to visualize the flow pattern and to measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels. Tests were performed in the range of inlet Reynolds number 25–60, mass flux 95–340 kg/m²s, and heat flux 80–330 kW/m².The flow visualization showed that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains. This process may be regarded as explosive boiling with periodic wetting and dryout.In the presence of two-phase liquid–vapor flow in the micro-channel, there are pressure drop oscillations, which increase with increasing vapor quality.This study shows strong dependence of the heat transfer coefficient on the vapor quality. The time when liquid wets the heated surface decreases with increasing heat flux. Dryout occurs immediately after venting of the elongated bubble.     

The identification of two distinct laminar to turbulent transition modes in pipe flows accelerated from rest

A study was undertaken to investigate transition in a pipe flow accelerated from rest. Experiments were carried out on a vertical tube under a constant head of liquid: flow was initiated by opening a solenoid valve. A wall shear stress probe used in the role of an event recorder identified two transition events, separated by the passage of a turbulent to laminar front and a period of laminar flow. Evidence suggests that the first comprises a laminar to turbulent interface arising from a natural stable/unstable front moving up the tube as local conditions become met, while the second is consequent upon the formation of a continuous turbulent structure carried down the tube from the inlet by the bulk flow. The paper provides a formal explanation of a phenomenon which is typical of that which is observed in starting pipe flows with a disturbed inlet.