Inlet Restriction Research Articles

Aero‐acoustic simulations of small scale electronic cooling blowers.

The noise emission of small radial blowers is important for the cooling of notebook systems. It has been demonstrated that airflow simulations for these radial blowers can be performed to predict the blower performance, even in the presence of inlet restrictions. The current paper presents an extension of this work to include predictions of the radiated acoustic noise. The calculated blower flow field is used as an input for a second stage aero‐acoustic prediction. This paper outlines two methods. The first method uses a free field approach based on the Ffowcs‐Williams and Hawkings analogy. The second approach constructs a rotating fan source, the magnitude of which is determined by integrating the pressure across a number of blade segments. The radiated field is then determined in a boundary element calculation, both for a free field and in the presence of the fan enclosure. Results for both methods are presented and compared to experimental data.

Experimental and numerical investigations on flashing-induced instabilities in a single channel

During the start-up phase, natural circulation BWRs (NC-BWRs) need to be operated at low pressure conditions. Such conditions favor flashing-induced instabilities due to the large hydrostatic pressure drop induced by the tall chimney. Moreover, in novel NC-BWR designs the steam separation is performed in the steam separators which create large pressure drops at the chimney outlet, which effect on stability has not been investigated yet. In this work, flashing-induced oscillations occurring in a tall, bottom heated channel are numerically investigated by using a simple linear model with three regions and an accurate implementation for estimating the water properties. The model is used to investigate flashing-induced instabilities in a channel for different values of the core inlet friction value. The results are compared with experiments obtained by using the CIRCUS facility at the same conditions, showing a good agreement. In addition, the experiments on flashing-induced instabilities are presented in a novel manner allowing visualizing new details of the phenomenon numerical stability investigations on the effect of the friction distribution are also done. It is found that by increasing the total restriction in the channel the system is destabilized. In addition, the chimney outlet restriction has a stronger destabilizing effect than the core inlet restriction. A stable two-phase region is observed prior to the instabilities in the experiments and the numerical simulations which may help to pressurize the vessel of NC-BWRs and thus reducing the effects of flashing instabilities during start-up.

Recent Work on Boiling and Condensation in Microchannels

Recent work on boiling of water and condensation of steam in single and parallel microchannels is reviewed in this paper. It is found that the amplitude and frequency of fluctuations of temperature and pressure during the unstable flow-boiling mode depend greatly on the inlet/outlet configurations and the exit vapor quality. By fabricating an inlet restriction on each microchannel or the installation of a throttling valve upstream of the test section, reversed flow of vapor bubbles can be suppressed resulting in a stable flow-boiling mode. Boiling heat transfer coefficient and pressure drop in microchannels under stable flow-boiling conditions are obtained. These data at high vapor qualities are found to be substantially different from the correlations obtained for flow-boiling in macrochannels. Microbubble emission boiling phenomena, which can defer the arrival of critical heat flux, exist in a partially heated Pyrex glass microchannel at sufficiently high heat flux and high inlet subcooling conditions. For condensation in a microchannel, transition from annular flow to slug/bubbly flow is investigated. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface. The location, at which the injection flow occurs, depends on the mass flux and the cooling rate of steam. Increase in steam mass flux, decrease in cooling rate, and microchannel diameter tend to enhance the instability of the condensate film on the wall, resulting in the occurrence of injection flow further downstream at increasingly high frequency. The pressure drop in the condensing flow increases with the increase in mass flux and quality or with decreasing microchannel diameter. The existing correlations for pressure drop and heat transfer of condensing flow in macrochannels overestimate the experimental data in microchannels.

Heat transfer by natural convection from a uniformly heated vertical circular pipe with different entry restriction configurations

Abstract Natural convection heat transfer for laminar air flow in a vertical circular pipe by using the boundary condition of constant wall heat flux in the ranges of RaL from 1.1 × 109 to 4.7 × 109 has been experimentally investigated. The experimental set up was designed for determining the effect of different configurations of restriction placed in the bottom position at the entry of the heated pipe on the surface temperature distribution, the local and average heat transfer coefficients. The apparatus was made from an electrically heated cylinder of a length 900 mm and inside diameter 30 mm. The inlet configurations included a circular pipe of the same diameter as the heated pipe but with lengths of 60 cm, 120 cm, sharp edge and bell mouth. It was found that the surface temperature along the pipe surface for the same heat flux would be higher for a circular restriction with length of 120 cm and would be smaller for a bell mouth restriction. The results show that the local Nusselt number Nux and average Nusselt number ( Nu ¯ L ) are higher for a bell mouth restriction and smaller for the 120 cm restriction. For all the inlet restrictions configurations, the results show that the Nusselt number values increases as the heat flux increases. Empirical correlations were proposed in the form of Log Nu L ¯ versus Log Ra L ¯ for each case investigated and a general correlation was obtained for all cases, which reveals the effect of the restriction existence on the natural convection heat transfer process in a vertical circular pipe.

An Investigation of Flashing-Driven Natural Circulation

Natural circulation driven boiling systems are extensively used for passive heat removal, such as in nuclear power plants. The flashing phenomenon and the associated flow instabilities in a natural circulation loop are reported here. An experimental facility is constructed with a heater, an adiabatic riser, a condenser and an adiabatic down comer to study the instability regions. The experimental geometry is modeled using RELAP5. At high power (i.e., at 6 kW) flashing instability is observed when the inlet sub cooling is within the range of 7° C to 12° C. It is also found that the presence of an inlet restriction does not help to suppress the flashing induced flow oscillation. A good comparison between the experimental and numerical results is observed and this loop can be used for validation exercises of commercial codes.

Inlet throttling effect on the boiling two-phase flow stability in a natural circulation loop with a chimney

Experiments have been conducted to investigate an effect of inlet restriction on the thermal-hydraulic stability. A Test facility used in this study was designed and constructed to have non-dimensional values that are nearly equal to those of natural circulation BWR. Experimental results showed that driving force of the natural circulation at the stability boundary was described as a function of heat flux and inlet subcooling independent of inlet restriction. In order to extend experimental database regarding thermal-hydraulic stability to different inlet restriction, numerical analysis was carried out based on the homogeneous flow model. Stability maps in reference to the core inlet subcooling and heat flux were presented for various inlet restrictions using the above-mentioned function. Instability region during the inlet subcooling shifted to the higher inlet subcooling with increasing inlet restriction and became larger with increasing heat flux.

Periodic flow excursion in an open two-phase natural circulation loop

Flow excursion instability in an open two-phase natural circulation loop was studied both experimentally and analytically. At high inlet subcooling, stable mode (continuous circulation) does not exist; instead, three different circulation modes such as periodic circulation (A), multimode circulation, and periodic circulation (B) appear at different heat flux conditions. The multimode circulation is represented by the combination of periodic circulations A and B and it is termed here as “periodic flow excursion”. The decrease in the inlet restriction and/or the increase in the exit restriction broadened the region of the flow excursion, and these aspects were summarized on the instability map of the heat flux vs. inlet subcooling. In addition, with the one-dimensional two-phase homogeneous equilibrium assumption, the region of flow excursion was obtained analytically, which fairly agrees with the experimental results.

Conjugate natural convection in a planar thermosyphon with multiple inlets—I. Velocity and temperature fields

A numerical investigation of flow ( Pr = 0.71) and temperature fields in a planar open loop thermosyphon is presented. The thermosyphon flow path is a modified ‘U’ shape with an upper and lower inlet in the right channel of the ‘U’. The flow was studied for different values of the governing parameters. These include the Rayleigh number ( Ra), the lower inlet width and the wall thermal conductivity. Results indicate the temperature field is independent of the inlet configuration at large Ra. The flow field is highly dependent on the lower inlet as recirculation at the outlet increases with inlet restrictions and boundary layer development at the heated wall. The wall thermal conductivity has large effects due to conduction resistance of the conducting walls and reduced natural convection effects.

空気圧シリンダの定速駆動に関する研究

In many industrial applications, open-loop control which uses on-off valves and restrictors are widely used for constant speed drive of pneumatic cylinders.This paper describes the conditions and characteristics of constant speed drive of pneumatic cylinders. First, we derived the conditions for the constant speed drive based on the equation of motion and equations of pressure change in cylinder chambers. It was shown that pressure in both chambers must be constant for constant speed drive. In case where air is exhausted with a choke velocity, the outlet restriction determines piston speed uniquely as well as the conventional meter-out circuit. Steadystate pressure of the forcing side is determined by the inlet restriction and this affects the velocity response when motion starts. In the conventional meter-out circuit, the inlet restriction is given only by the inlet valve and large overshoot occurs at the start of motion. To get a fast response with a small overshoot, the inlet restriction should be adjusted properly.The size of optimum inlet restrictions was calculated for various reference speeds and loads by using basic equations and a method to estimate the size of inlet restriction by using friction forces was discussed.Experiments were carried out and the effectiveness of inlet restriction for desirable motion was confirmed.

The Axisymmetric, Perfectly Flexible Foil Bearing With Porous Inlet Restrictor

The equations for the axisymmetric, perfectly flexible foil bearing are solved for the case of an externally pressurized foil with porous inlet restriction. The results are presented in graphical form for design use. For comparison, results for the axisymmetric, perfectly flexible foil bearing with discrete inlet restrictors are also given.