Effects Of Inlet Subcooling Research Articles

Study on two-phase flow instability of parallel helical tubes in steam generator of small modular reactors

The flow instability phenomenon within parallel helical tubes is experimentally studied. The oscillation characteristics of wall temperature are comprehensively discussed. In the experiment, the pressure of the test section is in the range of 2.7–6.0 MPa, the mass flux is in the range of 100–300 kg/m2s, the degree of inlet subcooling is in the range of 10–75 °C, and the inlet throttling is in the range of 82–328. This study reveals different effects of inlet subcooling. Specifically, increasing the inlet subcooling under high mass flux enhances the stability of parallel helical tubes. However, increasing the inlet subcooling under low mass flux and low subcooling weakens the stability. The oscillation period to mixture transit time ratio is small at high inlet subcooling and increases with decreasing subcooling. Furthermore, this study shows that the onset of wall temperature oscillation is closely related to the dryout phenomenon, and that the oscillation amplitude increases with increasing local quality. Finally, this study demonstrates that the flow instability can be accurately predicted using time-domain method. The simulation results show satisfactory accuracy, with the prediction errors of ± 15% for the instability threshold and oscillation period, and ± 20% for the wall temperature oscillation amplitude.

Effects of inlet subcooling on the flow boiling heat transfer performance of bi-porous mini-channels

Micro/mini-channel heat sinks have been viewed to be a potential option for the heat removal of electronics with rising thermal flux. A bi-porous mini-channel, fabricated through sintering copper woven tapes on channel surfaces, is developed in this paper for the sake of improving the flow boiling heat transfer. Two different pore structures exist in the copper woven tape: one is cavity constituted via copper strands, another one is crevice constituted via copper wires. The bi-porous mini-channel is multi-channel type, and the dimension of each single channel is 1.2 × 1.5 mm. Experimental studies,with various inlet subcoolings of 10.7, 13.7 and 16.7 °C, on the flow boiling performance of the bi-porous mini-channel were conducted, and a traditional mini-channel possessing planar surfaces was employed for comparison experiments. Working fluid R141b, inlet mass flux at 255 kg/m2·s and outlet pressure at 50 kPa (gage pressure) are adopted in the experiments. Boiling curve, heat transfer coefficient, temperature distribution and performance evaluation were investigated, combining with the visualization of bubble behaviors. The result demonstrates that the bi-porous mini-channel triggers the onset of nucleate boiling in the case of smaller heat flux conditions than the mini-channel with planar surface under different inlet subcoolings. The boiling hysteresis phenomenon is more obvious at higher subcooling for the mini-channel with planar surface, but it is eliminated in the bi-porous mini-channel under the same conditions. Under high heat flux conditions, the obvious inlet backflow phenomenon is observed and may deteriorate the heat transfer performance. The present study indicates that the bi-porous mini-channel possesses high heat transfer performance and temperature uniformity, which may meet the high demands of thermal management in fields such as battery cooling of electric vehicles.

Improvement of the critical heat flux correlation in a thermal-hydraulic system code for a downward-flow narrow rectangular channel

Several critical heat flux (CHF) correlations including the look-up table in the MARS code have been assessed for the prediction of CHF in a downward-flow narrow rectangular channel. For the assessment, we built an experiment database that covers pressures between 1.01 and 39.0 bar, gap sizes between 1.09 and 6.53 mm, mass fluxes up to 25,772 kg/m2s, and under one-sided and two-sided heating conditions. The results of the assessment showed that the Kaminaga correlation has the best overall prediction compared to others. However, because the correlation uses global variables, such as inlet and outlet subcooling and total heat transfer area, it is difficult to use in a system code. A new CHF correlation is then proposed by replacing the global variables in the Kaminaga correlation with local ones and adding correction factors to consider the effect of gap size, mass flux, and the number of heating walls. Additional correction factor is added to consider the effect of inlet subcooling. It is shown that the new one is better than the Kaminaga correlation and it is easy to implement to any system code.

Effects of Inlet Subcooling on Flow Boiling in Micro Channels - A Review

Effects of Inlet Subcooling on Flow Boiling in Micro Channels - A Review

Critical heat flux characteristics of R1234yf, R1234ze(E) and R134a during saturated flow boiling in narrow high aspect ratio microchannels

This experimental study investigated narrow and high aspect ratio multi-microchannels that were micro-machined in copper with thin separating walls during saturated flow boiling of refrigerants. The hypothesis was that these channels could increase the footprint critical heat flux and support the future development of thermal management systems for power electronics and other electronic packages. The measured footprint critical heat flux was as high as 678.5 W/cm2, which is twice as high as other investigations in the literature concerning saturated flow boiling of refrigerants. Two test samples were fabricated with a footprint area of (10 × 10) mm. The first had 25 rectangular channels (198 μm wide, 1167 μm high). The second had 17 rectangular channels (293 μm wide, 1176 μm high). The experimental investigation covered low-GWP replacement refrigerants (R1234yf, R1234ze(E)) as well as the well-examined R134a serving as a benchmark. A large data bank was obtained with 432 data points covering a wide range of typical inlet subcooling (1.3–14.7) K and mass fluxes (333–1260) kg/m2s as well as two nominal saturation temperatures (30 and 40) °C.The effect of inlet subcooling was found to be consistently significant at the higher mass fluxes. This was contradictory to other investigations that found moderate or insignificant effects. The result is suggested to be attributed to the two-phase stability induced by the upstream throttle valve, inlet orifices, isolated refrigerant in the inlet and outlet plenums as well as the short heated length causing higher importance of orifice to channel pressure drop importance. Finally, a new modified Katto and Ohno correlation including the effect of subcooling was proposed, achieving a 4.0% mean average error and predicting 93.3% of the data points within 10% error.

Effect of inlet subcooling on flow boiling in microchannels

The paper describes experimental work to assess the effect of inlet subcooling on the flow boiling characteristics of HFE-7200 in a multi-microchannel heat sink. The experiments were performed at a mass flux of 200 kg/m2 s and system pressure of 1 bar for inlet subcooling conditions of 5 K, 10 K and 20 K and wall heat flux between 25.9 and 180.7 kW/m2. The equivalent base heat flux was between 99.1 and 605.3 kW/m2. The microchannel evaporator had 44 channels which were 0.36 mm in width and 0.7 mm in depth. The thickness of the fin between the channels was 0.1 mm and the base area of the microchannels was 20 mm × 20 mm. Increasing inlet subcooling generally decreased two-phase pressure drop in the channels and delayed flow regime transitions in the heat sink. Flow instability was also found to be dependent on the degree of inlet subcooling. The extended subcooled region of the channels at higher degrees of subcooling resulted in lower average heat transfer coefficients in the heat sink, especially at low heat fluxes. The effect of inlet subcooling on local heat transfer coefficients was less pronounced in the saturated boiling region. The study verified the importance of the degree of subcooling when comparing heat transfer and pressure drop characteristics in microchannel heat sinks and when optimising the design of integrated thermal management systems for high heat flux electronic devices.

Experimental study on flow and heat transfer characteristics at onset of nucleate boiling in micro pin fin heat sinks

The micro pin fin heat sink has great application potential in the heat dissipation of high-power LED chips, phased array radars and semiconductor lasers. The onset of nucleate boiling (ONB) is a critical limit in designing heat dissipation systems. A simultaneous visualization and measurement study is conducted to investigate the ONB phenomenon of deionized water in circular, diamond, and oval micro pin fin heat sinks. Experiments are conducted under inlet subcooling of 29.6–49.5 °C, mass flux of 147.4–355.4 kg m−2 s−1 and heat flux of 4.8–55.8 W cm−2. Visualization result indicates that the ONB is always initiated at the surrounding region of a micro pin fin. The effects of additional nucleation sites caused by a micro pin fin and the temperature distribution of the surrounding region captured by an infrared camera can be used to explain the occurrence of the first bubbles. In this study, the effects of inlet subcooling, mass flux, and the cross-section shape of the micro rib on the ONB characteristics are investigated. Results show that the heat flux and wall superheat needed for the ONB increase as the mass flux and inlet subcooling increase. Moreover, the heat flux required for the ONB in the diamond and oval micro pin fin heat sinks is higher than that in the circular one. On the basis of the experimental data, the correlation of heat flux at the ONB in the micro pin fin heat sink is established, with 85.7% of the data falling within an error band of ±15%.

Wall boiling in a vertical annulus: Effect of inlet subcooling and mass flow rate

Numerical studies on low flow rate convection boiling in a vertical annulus has been carried out to predict effects of inlet subcooling and mass flow rate. The aspect ratio of vertical annulus is 352 while the annular gap is 3.5 mm. RPI wall boiling model is used for the development of present code and the results are verified with those available in literature. The results show that onset of significant void (OSV) can be delayed to achieve maximum heat transfer by increasing the liquid subcooling and liquid mass flow rate. The average Nusselt number increases almost linearly with increase in the mass flow rate as well as the inlet subcooling. At high heat flux, very high wall temperatures are observed with low subcooling and low mass flow rates. This should be avoided for enhanced safety.

Experimental and theoretical investigation on two types of density-wave oscillation in parallel twin narrow rectangular channels under forced circulation

Experimental and theoretical investigation on density-wave oscillation (DWO) was presented in this study. Experiment was performed on flow instability in parallel twin vertical rectangular channels under forced circulation. In the experiment, two types of DWOs were observed at different threshold condition. One type of DWOs occurred at low power with low exit quality, which was defined as Type I instability. The other type was observed at high power with high exit quality, which was defined as Type II instability. The effects of thermal parameters on two types of flow instability were investigated at pressure ranged from 3 to 8 MPa, mass velocity ranged from 200 to 500 kg/m2s, and inlet subcooling ranged from 40 to 140 °C. It was found that both types of DWOs were reduced by the increasing pressure and mass velocity, but the effects of inlet subcooling on Type I and Type II DWO were converse. Oscillation periods of Type I instability in this study were from 1.5 s to 2.5 s, and the period decreased with increasing mass velocity and decreasing inlet subcooling. Stability maps of Type I and Type II instabilities were also obtained, which showed that the unstable zones of Type I and Type II instabilities were discontinuous and the unstable zones of Type I instability were closer to liquid region. Finally, a new mathematical model of flow instability was developed, and the results of prediction for instability boundaries were obtained.

Effect of local phenomena on subcooled boiling oscillations in natural circulation boiling loop

In this paper, the authors report the estimations of bubble frequency and oscillation of local void fraction and their role in subcooled boiling oscillations in a low-pressure natural circulation boiling loop. The estimations primarily rely on the measurements of impedance using inductance L–capacitance C–resistance R (LCR) meter. The bubble frequencies determined from the impedance signals and the images are comparable. The effect of inlet subcooling on the bubble frequency and the oscillation of local void fraction has been studied and found to be remarkable. Based on the comparison of the oscillations of local void fraction, local pressure and loop flow rate, the effect of local dynamic phenomena on the system oscillations clearly demonstrates that the oscillations of void fraction trigger high-amplitude flow oscillations with a delay between the oscillations of void fraction and loop flow rate.

COMPARISON OF THE EFFECTS OF DIFFERENT TYPES OF TUBE INSERTS ON TWO-PHASE FLOW INSTABILITIES

Turbulence promoting inner elements such as rings, wings or winglets, twisted tapes, etc., is widely used in single- and two-phase flows. In this experimental work, the two-phase flow instabilities of forced convection boiling in a horizontal tube with ring- and twisted-tape-type inner elements inside are investigated. In order to analyze the two-phase flow of water, the experiments were conducted under a constant system pressure of 7.5 bar, and a constant heat input of 24 kW was applied to the outer surface of the circular test pipe with constant exit restriction. Three different inlet temperatures (Ti = 15, 25, and 35°C) were used to see the effect of inlet sub-cooling, and the mass flow rate was decreased from 110 to 20 g/s to obtain the characteristic curve. Two different pitch ratios for both the ring and twisted tape were evaluated in terms of stability. The results were evaluated in terms of appropriate variables for both the tube with insertions and the smooth tube. The analysis shows that the pressure drop increases with decreasing mass flow rate for all cases. The presence of the inner elements in the test pipe causes the boundaries of the oscillations to narrow in comparison with smooth tube. On the other hand, with the increase in the effective diameter of the test pipe the boundaries of the oscillations broaden. For each type of insertion, the increase of the pitch ratio makes the system less stable.

Numerical study of nuclear coupled two-phase flow instability in natural circulation system under low pressure and low quality

Based on the one-dimension two-phase drift flow model, the numerical simulation of two-phase flow stability characteristic on the test loop (HRTL-5) for 5 MW heating reactor (developed by the Institute of Nuclear and New Energy Technology of Tsinghua University, Beijing) is performed with and without coupled point neutron kinetics. The density wave oscillation instability is analyzed in the system under low pressure at 1.5 MPa and low steam quality less than 10%. The effect of inlet subcooling and heating flux on the system instability is simulated under the system pressure P sys = 1.5 MPa. The numerical results show that there exist two instability inlet subcooling boundaries at different heat flux. The numerical results show good agreement with the experimental results on HRTL-5 without consideration of point neutron kinetics. If coupled with point neutron kinetics, the system will exhibit little difference on instability boundaries from that without considering the nuclear characteristics. But the amplitude and the phase of the oscillation of the thermal hydraulic parameters of the system will be somehow affected in unstable zone if the system is coupled with point neutron kinetics.

Experimental investigation on diabatic flow of R-134a through spiral capillary tube

The present experimental investigation has been carried out to investigate the effects of various geometric parameters on the mass flow rate of R-134a through diabatic spiral capillary tube. In diabatic flow, the capillary tube is bonded with the compressor suction-line to form a counter-flow exchanger. The lateral type of diabatic capillary tube has been investigated in the present experimental study. The major geometric parameters investigated are capillary tube diameter, capillary tube length and coil pitch. In addition, effect of inlet subcooling on the mass flow rate through diabatic spiral capillary tube is also done. A comparison of the performance of diabatic spiral capillary tube has been made with adiabatic spiral capillary tube. Generalized empirical correlation for diabatic spiral capillary tube has also been proposed. It has been found that the predictions of the proposed correlation lie in the error band of ±7%.

Experimental research on flow instability in vertical narrow annuli

A narrow annular test section of 1.5mm gap and 1800mm length was designed and manufactured, with good tightness and insulation. Experiments were carried out to investigate characteristics of flow instability of forced-convection in vertical narrow annuli. Using distilled water as work fluid, the experiments were conducted at pressures of 1.0 ∼ 3.0MPa, mass flow rates of 3.0 ∼ 25kg/h, heating power of 3.0 ∼ 6.5kW and inlet fluid temperature of 20°C, 40°C or 60°C. It was found that flow instability occured with fixed inlet condition and heating power when mass flow rate was below a special value. Effects of inlet subcooling, system pressure and mass flow rate on the system behavior were studied and the instability region was given.

Effects of pool subcooling on boiling heat transfer in a vertical annulus with closed bottom

Effects of inlet subcooling on pool boiling heat transfer in a vertical annulus with closed bottom have been studied experimentally. For the test, a tube of 19.1 mm diameter and the water at atmospheric pressure have been used. Up to 50 K of pool subcooling has been tested and results of the annulus are compared with the data of a single unrestricted tube. The increase in pool subcooling results in much change in heat transfer coefficients. As the heat flux increases and the subcooling decreases, a deterioration of heat transfer coefficients is observed. The governing mechanisms are suggested as single-phase heat transfer and liquid agitation for the single tube while liquid agitation and bubble coalescence are the major factors at the bottom closed annulus.

The effect of inlet subcooling on the critical heat flux for downward flow with upstream compressibility

The effect of inlet subcooling on the critical heat flux for downward flow with upstream compressibility

The Effect of Inlet Subcooling on the Critical Heat Flux for Downward Flow With Upstream Compressibility

The Effect of Inlet Subcooling on the Critical Heat Flux for Downward Flow With Upstream Compressibility

Flashing Flow Discharge of Initially Subcooled Liquid in Pipes

The release rates of hot fluids stored under pressure during a pipe break or valve failure are a pertinent consideration in source term modeling as part of the environmental health and safety issue. This technical brief presents an analytical treatment for flashing discharge of a hot liquid in a pipe from a high pressure reservoir. Specifically, the liquid is initially subcooled relative to stagnation condition but superheated relative to ambient pressure. The effect of inlet subcooling, pipe length-to-diameter ratio, and physical property groups of the fluid can be readily assessed by the present generalized model. A criterion related to the amount of ''limiting'' subcooling is found where the discharge rate can be evaluated using a form similar to the incompressible pipe flow formula. Unlike other previously published works, the present solution yields a smooth transition from saturated inlet to subcooled conditions and reduces exactly to the nozzle flow limit at zero pipe length.

Numerical simulation of subcooled boiling and heat transfer in vertical ducts

A two-dimensional two-phase non-equilibrium model is presented to predict the void fraction distribution, flow and temperature fields for turbulent subcooled flow boiling at elevated pressures in heated vertical ducts (pipes and channels). The model is based on the conservation of mass, momentum, and energy equations for each of the two phases. Appropriate wall and interfacial conditions are formulated to close the problem. The non-equilibrium temperature and velocity distributions of the two phases and void fraction profiles show considerable variation along the radial and axial directions for the subcooled flow boiling cases considered. The effects of inlet subcooling, exit pressure and wall heat flux on the flow boiling behavior were predicted. The predicted radially averaged void fractions are compared with the experimental results available in the literature. The radial temperature distributions and void fraction profiles are also compared with available experimental measurements.

Effect of inlet subcooling on two-phase flow oscillations in a vertical boiling channel

In this work, results of experimental research to investigate the effects of heat transfer augmentation and inlet subcooling on two-phase flow instabilities are presented. For this purpose, a simple set-up was designed and built. The effect of inlet subcooling was investigated using different heat transfer surfaces and inlet temperatures at constant heat input of 415 W. Freon-11 has been used as the test fluid, and the experiments were carried out for six heater tubes having different heat transfer surfaces. Inlet temperatures were in the range of −9.8°C to 38°C. The results indicate that, in the range of present experiments, the system becomes more stable, that it's instability boundary moves into lower mass flow rates, with increase in the inlet subcooling. However, the amplitudes and the periods of the oscillations increase with increase in the inlet subcooling. For some of the tested surfaces there was a particular inlet subcooling, above and below which the system's stability decreased.