Vertical Narrow Rectangular Channel Research Articles

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.

Combined Forced and Free Convective Heat Transfer Characteristics in Narrow Vertical Rectangular Channel Heated from Both Sides

Combined forced and free convective heat transfer characteristics were experimentally investigated for water flowing under about 1 atm in a narrow vertical rectangular (750 mm long, 50 mm wide and 18 mm in gap) channel heated from both sides. Experiments were carried out for both downward and upward forced convective flows for Reynolds number Re of 4 × 101 ∼5 × 105 and Grashof number Gr of 5 × 104 ∼4 × 1011, where the distance x from the inlet of the channel is adopted as the characteristic length in Re and Gr. As the results, the following were revealed 1. (1) Heat transfer coefficients for the combined convective flow region can be expressed in simple forms with a dimensionless parameter Gr/Re 21/8 Pr 1/2 which is constituted by Gr, Re and Prandtl number Pr. 2. (2) From the above-mentioned simple expressions it can be identified that the combined convective heat transfer is significant for the condition of 2.5 × 10−4<Gr/(Re 21/8 Pr 2/1)< 1.8 × 10−3.3. (3) In the present experiment, little significant differences were observed in heat transfer characteristics between the combined convective flow region with upward forced convective flow and that with downward forced convective flow.

Boiling heat transfer in a narrow vertical rectangular channel. (Relationship between heat transfer coefficient and period of passing bubble)

An experimental study has been made of saturated boiling heat transfer for water and R 113 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, 20 mm wide) is located on one side at a position 150 mm from the entrance and bubbles are forcibly passed through it at designated period of 0.033 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 bubble above about 6×10-9 m2 (aT0 -9>m2, the heat transfer coefficients become independent of the period.

Combined forced and free convective heat transfer characteristics in narrow vertical rectangular channel heated from both sides.

Combined forced and free convective heat transfer characteristics were experimentally investigated for water flowing under about 1 atm in a narrow vertical rectangular (750 mm long, 50 mm wide and 18 mm in gap) channel heated from both sides. Experiments were carried out for both downward and upward forced convective flows for Reynolds number Re of 4 × 101 ∼5 × 105 and Grashof number Gr of 5 × 104 ∼4 × 1011, where the distance x from the inlet of the channel is adopted as the characteristic length in Re and Gr. As the results, the following were revealed 1. (1) Heat transfer coefficients for the combined convective flow region can be expressed in simple forms with a dimensionless parameter Gr/Re 21/8 Pr 1/2 which is constituted by Gr, Re and Prandtl number Pr. 2. (2) From the above-mentioned simple expressions it can be identified that the combined convective heat transfer is significant for the condition of 2.5 × 10−4<Gr/(Re 21/8 Pr 2/1)< 1.8 × 10−3. 3. (3) In the present experiment, little significant d...

Boiling heat transfer in a narrow vertical rectangular channel.

An experimental study has been made of saturated boiling heat transfer in a narrow space (S=1.4-3.2 mm) at atmospheric pressure, in which the vertical heated surface (10 mm long, 20 mm wide) is located on one side and bubbles are forcibly infected at designated periods above 0.5 sec. The experiment shows that at low heat fluxes, the heat transfer depends on the bubbles injected but at high heat fluxes, it depends on a fraction of a lump of the bubbles generated on the heated surface in place of the bubbles injected. In addition, heat transfer coefficients are proportional to the reciprocal of the square root of the period at which the bubble infected at low heat fluxes, or the lump of bubbles at high heat fluxes, passes through the heated surface.

Experimental study of incipient nucleate boiling in narrow vertical rectangular channel simulating subchannel of upgraded JRR-3.

Experiments were carried out with a vertical rectangular channel simulating a sub-channel of the upgraded JRR-3 fuel element, in order to investigate the validity and the error of the correlations predicting the superheat at the onset of nucleate boiling. These correlations were used in the core thermal-hydraulic design of the upgraded JRR-3. As the results, the following were made clear: (1) The existing Bergles-Rohsenow correlation gives a good prediction for the relationship of heat flux vs. superheat at the onset of nucleate boiling, with the error of about 1 K against the lower limits of the measured superheat. (2) There are no significant differences in the characteristics of the relationship of heat flux vs. superheat at the onset of nucleate boiling between upflow and downflow. (3) There are no significant differences in the histories of relationship of heat flux vs. superheat from the forced convection single-phase flow to the subcooled boiling between increasing heat flux and decreasing heat flu...

Experimental Study of Incipient Nucleate Boiling in Narrow Vertical Rectangular Channel Simulating Subchannel of Upgraded JRR-3

Experiments were carried out with a vertical rectangular channel simulating a sub-channel of the upgraded JRR-3 fuel element, in order to investigate the validity and the error of the correlations predicting the superheat at the onset of nucleate boiling. These correlations were used in the core thermal-hydraulic design of the upgraded JRR-3. As the results, the following were made clear: (1) The existing Bergles-Rohsenow correlation gives a good prediction for the relationship of heat flux vs. superheat at the onset of nucleate boiling, with the error of about 1 K against the lower limits of the measured superheat. (2) There are no significant differences in the characteristics of the relationship of heat flux vs. superheat at the onset of nucleate boiling between upflow and downflow. (3) There are no significant differences in the histories of relationship of heat flux vs. superheat from the forced convection single-phase flow to the subcooled boiling between increasing heat flux and decreasing heat flux, with little overshoot of superheat at the onset of nucleate boiling both in the upflow and in the downflow.

Experimental Study of Differences in Single-Phase Forced-Convection Heat Transfer Characteristics between Upflow and Downflow for Narrow Rectangular Channel

The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25 mm in gap and 750 mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25 mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for down-flow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow.

Experimental study of differences in single-phase forced-convection heat transfer characteristics between upflow and downflow for narrow rectangular channel.

The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25 mm in gap and 750 mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25 mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for down-flow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow.

Experimental study of steady and transient natural convection heat transfer of mercury

An experimental investigation of transient and steady-state natural convection in a narrow vertical rectangular channel following a step-change in uniform wall-heat-flux is presented. The construction and instrumentation for two test sections are described. These test sections formed a rectangular channel 15.2×2.54×25.4 cm and consisted of: 1) both 15.2×25.4 cm faces heated uniformly by constant radiant heat flux with mercury as the fluid, and 2) the same boundary conditions as 1 but lead was used to thermally model the mercury.