Arunkumar Sridharan

An active control strategy for reduction of pressure instabilities during flow boiling in a microchannel

Flow instabilities during boiling in a microchannel have been well studied and documented in the literature. However, passive/active ways to reduce their magnitude are less well explored. In this paper, we examine the effect of external pulsations on flow with respect to reduction in pressure instability during boiling. Both the frequency and the amplitude of superimposed pulsations are varied and the study is performed for different mass flow rates and heat flux values. It is observed that the pressure fluctuations reduce significantly at high frequencies for all the cases studied; however, small frequencies of external pulsations may or may not be helpful. Interestingly, the average pressure is also found to reduce in certain cases. Flow visualization reveals that the flow switches from bubbly to annular when external pulsations are applied, which leads to this reduction in the pressure fluctuation. These results demonstrate that it may be possible to simultaneously reduce both mean and fluctuating pressure drops by carefully choosing the operating parameters.

Flow map and measurement of void fraction and heat transfer coefficient using an image analysis technique for flow boiling of water in a silicon microchannel

The present work focuses on the generation of the flow regime map for two-phase water flow in microchannels of a hydraulic diameter of 140 µm. An image analysis algorithm has been developed and utilized to obtain the local void fraction. The image processing technique is also employed to identify and estimate the percentage of different flow regimes and heat transfer coefficient, as a function of position, heat flux and mass flow rate. Both void fraction and heat transfer coefficient are found to increase monotonically along the length of the microchannel. At low heat flux and low flow rates, bubbly, slug and annular flow regimes are apparent. However, the flow is predominately annular at high heat flux and high flow rate. A breakup of the flow frequency suggests that the flow is bistable in the annular regime, in that at a fixed location, the flow periodically switches from single-phase liquid to annular and vice versa. Otherwise, the occurrence of three regimes—single-phase liquid, bubbly and slug are observed. These results provide several useful insights about two-phase flow in microchannels besides being of fundamental interest.

Investigations on two-phase heat exchanger for mixed refrigerant Joule-Thomson cryocooler

The design of the recuperative heat exchanger used to pre-cool the refrigerant mixture prior to J-T expansion is crucial for the efficient operation of the mixed refrigerant Joule- Thomson (MR J-T) cryocooler. The multi-component non-azeotropic refrigerant mixture undergoes boiling and condensation heat transfer simultaneously in the heat exchanger. Therefore, it is important to analyze the performance of the heat exchanger in terms of temperature distribution with respect to the mixture of gases used. In the present work, temperature measurements are carried out at the ends of the heat exchanger for high pressure stream, while eight sensors are installed at equal distance along the length of heat exchanger to measure temperature of low pressure stream. The paper reports variation in heat transfer coefficient along the length of the heat exchanger. The variation is discussed with respect to temperature distribution across the length and changes in thermo-physical properties of the gas mixture.

Effect of chemical cleaning on steam generator tube performance

Boiling experiments were performed on new, chemically cleaned, and fouled steam generator tubes to determine the heat transfer performance of each. It was found that the heat transfer performance of the fouled tube was the best, followed by the chemically cleaned tube. The performance of the new tube was the worst. Scanning electron microscope (SEM) photographs of the boiling surfaces were taken to identify differences in surface characteristics. Results revealed the presence of significant amounts of porous deposits on the surface of the fouled tube that provided ample nucleation sites for boiling. Chemical cleaning removed most of the deposits such that the boiling performance of the cleaned surface was degraded. The new tube was very smooth and there were relatively fewer nucleation sites as evidenced in the SEM photographs. Available correlations were used to predict the heat flux for a given wall superheat and were compared with the experimental data.

Experimental Study of Water Boiling in Microchannel

With the reduction in size of electronic devices, the problem of efficient cooling is becoming more and more severe. Boiling heat transfer in microchannels is fast emerging as a promising solution to the problem. In the present work, microchannels were fabricated on a silicon wafer. A chrome-gold micro-heater was integrated and characterized on the other side of the wafer. The change in resistance of the micro-heater in the temperature range of 20 °C – 120 °C was found to be within 10%. Deionized water was used as working fluid in microchannel. The single-phase pressure drop across the microchannel was found to increase linearly with increasing flow rate in confirmation with conventional laminar flow theory. Also, the pressure drop decreases with an increase in heat input due to a reduction in viscosity. The study was extended to two phase flow with flow rate and heat flux as the control parameters. The onset of two phase flow, at a given heat flux, with a decrease in flow rate, can be identified by the departure of linear pressure drop to non-linearity; this point was also confirmed through visual observation. In two-phase region of flow, pressure drop was found to increase initially, passes through a maximum and then decreases, with a decrease in flow rate. The experiments are performed for several heat fluxes. Both the onset of two phase and maximum pressure drop in the two phase region shifts to higher flow rates with an increase in heat input. Such detailed experimental results seem to be missing from the literature and are expected to be useful for modeling of boiling heat transfer in microchannels. Another pertinent observation is presence of instability in two-phase flow. It was found that at higher flow rate and heat flux instability in two-phase flow was more. An attempt to record these instabilities was made and preliminary data on their frequency will be presented. This study may help to choose suitable operating conditions for a microchannel heat sink for use in electronics cooling.Copyright

Numerical Investigation on Orthogonal Impingement of Circular Air Jet on a Heated Flat Plate at Low Jet Plate Spacing

Jet impingement cooling has been studied extensively as this finds applications in the areas of reactor safety, electronic cooling etc. The convective heat transfer process between the circular air jet impingement on uniformly heated flat plate is studied numerically. In this numerical study, 3-D simulations are carried out in Fluent 14.5 to investigate the effect of Reynolds number, distance between nozzle exit and the plate on the heat transfer characteristics. Standard κ-e, SST κ-ω, Standard κ-ω, V2F turbulence models have been studied for orthogonal jet impingement in this work. For Z/d ≥ 0.5, V2F model is best suited. Reynolds number of 12000, 20000, and 28000 has been studied. A typical feature of circular air jet impingement at low jet plate spacing is occurrence of secondary peak in Nusselt number variation along radial direction over the plate. V2F model correctly predicts the secondary peak in Nusselt number variation over the plate. Other models fail to predict the secondary peak which is of significant importance in air jet impingement at low jet-plate spacing. V2F model do not use wall functions. A fine grid at the surface of the wall is necessary to capture the flow behavior near the wall.

Numerical Investigation on an Obliquely Impinging Circular Air Jet on a Heated Flat Plate at Small Jet Plate Spacing

Jet impingement cooling has been studied extensively as this finds applications in the areas of reactor safety, electronic cooling etc. In practical applications, orthogonal jet impingement is a very ideal situation and the jet may hit the surface at some angle causing local uneven Nusselt number distribution over the surface. In this numerical study, 3-D simulations are carried out in Fluent 14.0 to investigate the effect of Reynolds number, distance between nozzle exit and the plate on the heat transfer characteristics. Standard κ-e with standard wall functions and enhanced wall functions, SST κ-ω, Standard κ-ω, V2F turbulence models have been studied for orthogonal jet impingement in this work. This study has been extended to inclined jet impingement. At jet plate spacing of Z/d ≤ 0.5, SST κ-ω turbulence model predicts Nusselt number variation satisfactorily while for Z/d ≥ 0.5 V2F turbulence model is best suited. In this work, jet plate spacing of Z/d = 2, Reynolds number of 28000 and 40000 has been studied. At small jet plate spacing and inclined jet impingement, the flow behavior over the plate changes. Profile of constant Nusselt number elongates along the downhill side of the plate. Contours of high Nusselt number compresses towards uphill side of the plate. Maximum heat transfer rate is from the downhill side of the plate and location of point of maximum heat transfer shifts towards “uphill” side of the plate. This shift in point of maximum heat transfer depends on “angle of jet impingement”.

Prediction of temperature distribution in heat exchanger for mixed refrigerant Joule–Thomson cryocooler

The overall performance of a mixed refrigerant Joule-Thomson (MR J–T) cryocooler depends on the efficiency of the heat exchanger used to cool the refrigerant prior to J-T expansion. The multi-component refrigerant mixture gets condensed and evaporated simultaneously at different pressures in the recuperative heat exchanger which is responsible to increase its performance. However, at present, the design of such heat exchanger is difficult due to lack of experimental heat transfer data.

Investigation of pressure drop in capillary tube for mixed refrigerant Joule-Thomson cryocooler

A capillary tube is commonly used in small capacity refrigeration and air-conditioning systems. It is also a preferred expansion device in mixed refrigerant Joule-Thomson (MR J-T) cryocoolers, since it is inexpensive and simple in configuration. However, the flow inside a capillary tube is complex, since flashing process that occurs in case of refrigeration and air-conditioning systems is metastable. A mixture of refrigerants such as nitrogen, methane, ethane, propane and iso-butane expands below its inversion temperature in the capillary tube of MR J-T cryocooler and reaches cryogenic temperature. The mass flow rate of refrigerant mixture circulating through capillary tube depends on the pressure difference across it. There are many empirical correlations which predict pressure drop across the capillary tube. However, they have not been tested for refrigerant mixtures and for operating conditions of the cryocooler. The present paper assesses the existing empirical correlations for predicting overall pres...