Vertical Narrow Rectangular Channel Research Articles

Experimental investigation on flow and heat transfer characteristics of single-phase flow with simulated neutronic feedback in narrow rectangular channel

Abstract Single phase forced convection flow and heat transfer of deionized water in horizontal and vertical narrow rectangular channel with gap size of 1.8 mm were experimentally investigated. The integration of thermal hydraulic hardware tests with simulated neutronic feedback has been carried out in this experiment. The results indicated that the transition Reynolds number from laminar to turbulent region in the narrow rectangular channel was about 1500–2300. Compared with the conventional channels, the friction factor became larger in narrow rectangular channel. Nusselt number in narrow rectangular channel was higher than that in conventional channels in laminar region while it was lower in turbulent region. Furthermore, the variations of power, heating surface temperature, inlet/outlet temperature, Re number and Nu number in transient process were investigated with the consideration of neutronic feedback. It indicated that both the quantity and rate of introduced reactivity affected the peak of power. The quantity of power raise was not directly proportional to the introduced reactivity. The delay effect of wall and fluid temperature to the power raise could not be ignored. The temperature peak came later than the power peak. The variations of Re and Nu number were considerable due to the appreciable power raise. In addition, there was no significant difference on the variations of friction factor and Nu number versus Re number between transient and steady state.

Flow Patterns Transition Criteria from Bubbly to Slug Flow during Flow Boiling in Confined Vertical Narrow Rectangular Channels

To investigate new features of flow patterns transition during flow boiling in confined vertical narrow rectangular channels, both experiments and theoretical analysis were carried out in the present study. When the channel size was smaller than the bubble departure diameter, a new flow pattern was observed and defined as “confined bubbly flow”. According to the relative size between bubble departure diameter and channel size, two groups of flow pattern transition criteria were developed by using modified drift flux model, with taking features of flow boiling and narrow confinement into account. Satisfactory agreement was obtained by compare model prediction with experiment results. However, further verification and modification are still needed for wider applications.

Theoretical study on the characteristics of critical heat flux in vertical narrow rectangular channels

Abstract A mathematical separated flow model of annular upward flow has been developed to predict the critical heat flux (CHF) in uniformly heated vertical narrow rectangular channels. The theoretical model is based on fundamental conservation principles: the mass, momentum, and energy conservation equation of the liquid film and the momentum conservation equation of the vapor core together with a set of closure relationships. The comparisons between the theoretically predicted results and the experimental data demonstrate that a good agreement between them has been achieved. With the applications of the present model, the influences of several parameters (such as system pressure, inlet mass flux and channel size) on the CHF in rectangular channels are analyzed, and the CHF is compared with that in annular channels.

Experimental investigation of natural convection heat transfer in narrow vertical rectangular channel heated from both sides

The heat transfer characteristics of the natural convection regime through a vertical rectangular channel simulating a cooling channel of typical material testing reactor have been experimentally investigated. Experiments are performed on demineralized water as coolant passing under atmospheric pressure through narrow rectangular channel of 80 cm length, 7 cm width and 2.7 mm gap thickness under different heat fluxes ranging from 2.7 kW/m 2 to 26.8 kW/m 2 that covers almost all possible heat fluxes in the single-phase liquid. The measured local Nusselt number values are compared with the predictions of two correlations for both natural and combined natural and forced convection regimes respectively. The comparison shows the need for developing new correlation to represent the obtained experimental results and to be utilized for the prediction of the local Nusselt number in thermal–hydraulics and safety analysis of the reactor. The developed correlation fits the experimental data within 5.8% relative standard deviation.

Photographic Study on Two-Phase Flow Patterns of Water in a Single-Side Heated Narrow Rectangular Channel

Abstract Visualized experimental observation on flow patterns during flow boiling of water under single-side heated and fluid-inlet subcooled conditions in a vertical narrow rectangular channel with the cross section of 40×3 mm2 have been carried out. Four discernible flow patterns, which names dispersed bubbly, coalesced bubbly, churn flow, and annular flow are observed. Flow visualization in two dimensions of two-phase flow patterns for narrow rectangular channel, which provided clearer evidence to distinguish flow patterns, have been performed. Based on the experimental results, a flow pattern map for single-side heated narrow rectangular channel has been developed and then compared with the exiting maps and flow transition criteria.

Thermal and hydrodynamic characteristics of forced and mixed convection flow through vertical rectangular channels

This paper presents experimental and numerical studies for the case of turbulent forced and mixed convection flow of water through narrow vertical rectangular channel. The channel is composed of two parallel plates which are heated at a uniform heat flux, whereas, the other two sides of the channel are thermally insulated. The plates are of 64 mm in width, 800 mm in height, and separated from each other at a narrow gap of 2.7 mm. The Nusselt number distribution along the flow direction normalized by the Nusselt number for the case of turbulent forced convection flow is obtained experimentally with a comparison with the numerical results obtained from a commercial computer code. The quantitative determination of the nor- malized Nusselt number with respect to the dimension-less number Z = (Gr/Re21/8Pr0.5) is presented with a comparison with previous experimental results. Qualitative results are presented for the normalized temperature and velocity profiles in the transverse direction with a comparison between the forced and mixed convection flow for both the cases of upward and downward flow directions. The effect of the axial locations and the parameter Gr/Re on the variation of the normalized temperature profiles in the transverse direction for both the regions of forced and mixed convection and for both of the upward and downward flow directions are obtained. The normalized velocity profiles in the transverse directions are also determined at different inlet velocity and heat fluxes for the previous cases. It is found that the normalized Nusselt number is greater than one in the mixed convection region for both the cases of upward and downward flow and correlated well with the dimension-less parameter Z for both of the forced and mixed convection regions. The temperature profiles increase with increasing the axial location along the flow direction or the parameter Gr/Re for both of the forced and mixed convection regions, but this increase is more pronounced in the case of the mixed convection flow. For the forced convection region, the velocity profile depends only on Re with no difference between the upward and downward flow directions. Whereas, for the case of mixed convection flow, the velocity profile depends on the parameter Gr/Re with a main difference between upward and downward flow. These results are of great importance for any research reactor using plate type fuel elements or for any engineering application in which mixed convection flow through rectangular channel is encountered. .

Flow regime transition criteria for upward two-phase flow in vertical narrow rectangular channels

In relation to the cooling system of high performance microelectronics, a high power research reactor with plate type fuels and plasma facing components of a fusion reactor, study of two-phase flow in a narrow rectangular channel has been paid considerable attention, recently. For the two-fluid model, direct geometrical parameters such as the void fraction should be used in flow-regime criteria. From this point of view, flow-regime transition criteria for vertical upward flows in narrow rectangular channels have been developed considering the mechanisms of flow-regime transitions. The basic concept of the present modeling followed the Mishima and Ishii model for vertical upward two-phase flows in round tubes. Newly developed criteria have been compared with the existing experimental data for air–water flows in narrow rectangular channels with the gaps of 0.3–17 mm. The present criteria showed satisfactory agreements with those data. Further comparisons with data for steam–water in a rectangular channel at relatively high system pressures have been made. The results confirmed that the present flow-regime transition criteria could be applied over wide ranges of parameters as well as to boiling flow.

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel (Change in heat transfer along flow)

The objective of this paper is to make clear how heat transfer coefficient changes along the flow with the passing bubbles through a narrow vertical rectangular channel (20 mm wide, 2 mm deep and 450 mm long). The experiments were done using subcooled water of 80, 60, and 40 K at atmospheric pressure in which the air bubbles were injected into the channel at a designated period from 0.125 to 1.0 s and their length was controlled to be equal to 0.03, 0.02, and 0.01 m. The experiment shows that the heat transfer coefficients decrease along the flow and then reach a constant value beyond a certain distance from the leading edge of the heated surface where the flow becomes fully developed in both the velocity and the thermal conditions. Under the fully developed conditions, the heat transfer coefficients are predicted well by the existing theoretical analysis in which both the convective term and evaporation on the interface are ignored.

Enhancement of Heat Transfer Due to Bubbles Passing through a Narrow Vertical Rectangular Channel. Change in Heat Transfer along the Flow.

The objective of this paper is to clarify how the heat transfer coefficient changes along the flow due to bubbles passing through a narrow vertical rectangular channel (20 mm wide, 2 mm clearance and 450 mm long). The experiments are performed using subcooled water of 80, 60 and 40 K at atmospheric pressure in which the air bubbles are injected into a channel at designated intervals from 0.125 to 1.0 sec and the bubble length is controlled to be equal to 0.03, 0.02 and 0.01 m. The experiments show that the heat transfer coefficients decrease along the flow and then reach constant values beyond a certain distance from the leading edge the heated surface where the flow reaches a fully developed state in both the velocity and the thermal conditions. Under the fully developed conditions, the heat transfer coefficients are predicted well by the existing theoretical analysis in which both the convective term and evaporation on the interface are ignored.

Experimental Study of Falling Water Limitation under a Counter-Current Flow in a Vertical Rectangular Channel : 1st Report, Effect of Flow Channel Configuration and Introduction of CCFL Correlation

Counter-current-flow limitation (CCFL) experiments were carried out for both vertical rectangular channels and vertical circular tubes varying in size and in configuration of their cross sections to clarify CCFL characteristics in the vertical rectangular channels. Quantitative understanding of critical heat flux (CHF) in a narrow vertical rectangular channel is required for the thermohydraulic design and safety analysis of research nuclear reactors in which flat-plate-type fuel is employed. Critical heat flux under downward low velocity is closely related to falling water limitation under a counter-current flow. Experimental results showed that the equivalent hydraulic diameter de, i.e., the width, and gap of the channel play an important role in determining the CCFL characteristics of a rectangular channel. However, a significant influence of channel length on CCFL characteristics was not observed in the ranges investigated. Using new dimensionless parameters, the authors propose a correlation for predicting the relationship between upward air velocity and downward water velocity based on the present experimental results.

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel

A theoretical analysis has been made of heat transfer enhancement due to bubbles passing through a narrow vertical channel. The mechanism of enhancement for a constant heat flux is: heat is first transported by a latent heat of evaporation on the bubble interface covering the heated surface and is followed by sensible heating of the succeeding liquid. The latent heat transport is calculated using an integral method and the sensible heating is determined exactly. For low heat fluxes, the calculated heat transfer coefficients are in fairly good agreement with the experiments; at high heat fluxes, however, the calculated values are approximately half the experimental values, since flow along the heated surface is not taken into account.

A CHF characteristic for downward flow in a narrow vertical rectangular channel heated from both sides

A new correlation for the countercurrent flow limitation (CCFL) in vertical rectangular channels was applied to the prediction of critical heat flux (CHF) for downward flow in a vertical rectangular channel heated from both sides. The new correlation was based on CCFL experiments carried out with a two-phase flow system of air and water under 1 atm with a superficial air velocity range of 1–17 m/s. The CHF experiments were carried out with an inlet water subcooling range of 25–75 K and an inlet water mass flux range of 2–600 kg/m 2s under about 1 atm for a 750 mm long, 50 mm wide and 2.25 mm gap flow channel and a 375 mm long, 50 mm wide and 2.80 mm gap flow channel which were heated from both sides. The comparison between the prediction based on the new correlation for the CCFL and the CHF experimental results could provide a good quantitative understanding of CHF characteristics, which is required for the thermal-hydraulic design and safety analysis of nuclear research reactors in which the downward flow is adopted for core cooling with flat-plate-type fuel. The role of the aspect ratio of the rectangular channel was evident in both the CCFL and CHF characteristics for countercurrent flow and it was strongly implied that the CHF for downward flow is at a minimum under the flooding condition in the case of large inlet water subcooling and when the inlet downward water mass flux is greater than that under the flooding condition in the case of small inlet water subcooling.

Combined Forced and Free Convective Heat Transfer Characteristics in a Narrow Vertical Rectangular Channel with 2.5 mm in Gap Heated from Both Sides

Combined Forced and Free Convective Heat Transfer Characteristics in a Narrow Vertical Rectangular Channel with 2.5 mm in Gap Heated from Both Sides

Experimental study of critical heat flux in a narrow vertical rectangular channel. Comparison between uniform heat flux case and nonuniform heat flux case.

Quantitative understanding of critical heat flux (CHF) in a narrow vertical rectangular channel is required for the thermohydraulic design and safety analysis of research nuclear reactors in which flat-plate-type fuel is employed. Experiments were carried out on CHF for both upward flow and downward flow, with nonuniform heat flux simulating a subchannel in the fuel element of the research nuclear reactor JRR-3. Investigation of experimental data showed that the CHF scheme proposed by the authors for upward flow and downward flow is applicable, not only to the uniform heat flux case, but also to the nonuniform heat flux case.

Enhancement of heat transfer due to a bubble passing through a narrow vertical rectangular channel. Effect of subcooling on heat transfer.

An experimental study has been made of the enhancement of heat transfer for water at a subcooling of 70, 40, 20, and 10 K and for ethanol at a subcooling of 60 and 30 K in a narrow vertical rectangular channel (20 mm wide, 2 mm space and 200 mm long) through which air bubbles are forcibly passed at a designated period of 0.1 to 10 sec. The length of the bubble can be controlled to be equal to 30, 20 and 10 mm in length. The experiment shows that the heat transfer coefficients at the subcooling of 40 and 70 K are determined only by the period of the passing bubble within the experimental range. This result means that the enhancement of the heat transfer due to the passing bubble is mainly made by sweeping away the thermal layer on the heated surface with its bubble.

Enhancement of Heat Transfer due to Bubble Passing through a Narrow Vertical Rectangular Channel

Enhancement of Heat Transfer due to Bubble Passing through a Narrow Vertical Rectangular Channel

Combined forced and free convective heat transfer characteristics in a narrow vertical rectangular channel with 2.5mm in gap heated from both sides.

Heat transfer characteristics of mainly combined forced and free convective flow in a vertical rectangular flow channel with a gap of 2 5 mm, which was quite narrow compared with those investigated in previous experiments, were studied experimentally for water. As a result, the following heat transfer characteristics were made clear, using a non-dimensional parameter Gr x /Re 21 x /8 Pr 1/2 similarly to the case for the 18 mm gap which was already reported by the authors. (1) When the Gr x /Re 21 x /8 Pr 1/2 is less than 10−4, both upward flow and downward flow show the nature of forced convective heat transfer. (2) When the Gr x /Re 21 x /8 Pr 1/2 is between 10−4 and 10−2, heat transfer coefficients for both upward flow and downward flow are higher than any of those predicted by the previous correlations for turbulent forced convection along a flat plate and turbulent free convection along a vertical flat plate. This is, differently from the case of 18 mm gap, due to the effect of the acceleration of mai...

The enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel. The effect of bubble length and clearance of channel on heat transfer.

An experimental study has been made of the saturated boiling heat transfer for water and R 113 in a narrow vertical rectangular channel (20 mm wide and 200 mm long) at atmospheric pressure, in which the vertical heated surface (10 mm long, 20 mm wide) is located on one side at a position 150 mm from the entrance and bubbles are forcibly passed through it for a designated period of 0.033 to 1.0 sec. The clearance of the channel can be changed from s=2 mm to s=7 mm, and thus the size of the bubble can be controlled independently of the clearance to be equal to 30, 20, and 10 mm in length. The experiment shows that the length of the passing bubble significantly enhances the heat transfer and the clearance has a weak effect on its enhancement when a bubble of the same length passes through it.

Upper limit of enhancement of boiling heat transfer in a narrow vertical rectangular channel due to passing bubbles

An experimental study has been made of saturated boiling heat transfer for water and R113 in a narrow vertical rectangular channel (2 mm space, 20 mm wide, and 200 mm long) at atmospheric pressure, in which the vertical heated surface (10 mm long and 20 mm wide) is located on one side at a position of 150 mm from its entrance and bubbles are forcibly passed through it at a designated period from 0.33 to 1.0 sec. The experiment shows that the heat transfer coefficients are increased by the bubble passing through the heated surface for the value of thermal diffusivity,a, times period, T0, of the passing bubbles above about 6×10−9 m2 (a T0>6×10−9 m2) while fora T0< 6×10−9 m2, the heat transfer coefficients become independent of the period and the effectiveness of the enhancement of the heat transfer owing to the passing bubble disappears.