Experiments on subcooled flow boiling have been conducted using water in a rectangular flow channel. Similar to the coolant channel in internal combustion engines (IC engines), the flow channel in this experiment was asymmetrically heated. Bubble images were captured using a high speed camera from the side view of the channel. The experimental conditions in terms of bulk temperature, bulk velocity, pressure and heat flux ranged from 65°C–75°C, 0.25 m/s–0.75 m/s, 1–1.7 bar and 490 kW/m^{2}–700 kW/m^{2}, respectively. On the basis of these tests, a statistical analysis of the bubble size has been conducted considering a population of 1400 samples. It has been found that the mean Sauter bubble diameter increases with the decrease of subcooling, bulk velocity, pressure and increased heat flux. A modified correlation has been finally proposed to predict the mean Sauter bubble diameter under subcooled flow boiling conditions upstream of the onset of significant void, which shows good accuracy with the experimental results.

Subcooled flow boiling has attracted drastic attention since its advantage of high heat transfer capacity with low temperature difference [

Multiple factors have been found to affect the subcooled flow boiling heat transfer. For instance, operation conditions (i.e., degree of subcooling, bulk flow velocity and pressure) [

Bubble departure diameter is the diameter when bubble leaves its nucleation site. And this diameter is considered as the most important parameter and widely adopted in CFD simulations for calculation of evaporation heat flux in subcooled flow boiling conditions [

Bubble lift-off diameter defines the diameter when bubble detaches from the heated surface. And this diameter equals to bubble departure diameter when bubble detaches from its nucleation site without sliding. It is pointed out that the bubble lift-off diameter should be more appropriate for predicting the heat transfer process considering the effect of bubble sliding [

Maximum bubble diameter is considered as the upper limit the bubble can reach. Ünal [

The mean Sauter bubble diameter is defined to calculate the interfacial area concentration in two-fluid models in determining the interfacial momentum and heat transport. Zeitoun et al. [

However, the correlation for predicting mean sauter bubble diameter by Zeitoun et al. [

Besides the differences among experiment layouts and operating conditions, visualization process method might still lead to different conclusions. Generally, bubble pictures are taken from the side view, top view or both. Yoo et al. [

In this paper, a visualization experiment was conducted for flow boiling in a horizontal rectangular channel. Bubbles on heating surface were captured from the side view. The experimental range for bulk temperature, bulk velocity and pressure were 65°C–75°C, 0.25 m/s–0.75 m/s and 1–1.7 bar, respectively. Statistical analysis was conducted on these conditions and the effect of operation parameters on bubble behavior was investigated. And a modified correlation for predicting mean Sauter bubble diameter in the region upstream of OSV was proposed.

An experiment system was built to investigate the bubble distribution on heating surface of subcooled flow boiling. This system consists of two parts: Flow circulating section which was designed for accomplishing various subcooled boiling conditions and imaging sub-system to obtain photos of bubbles, as shown in

The closed flow circulating system consists of storage tank, pump, flowmeter, heating apparatus and rectangular test section. The storage tank was designed with the capacity of 250 L for keeping the bulk temperature stable and four immersion heaters with total maximum power of 8 kW are installed on the bottom of it. Water was used as the working fluid. A PID controller, receiving temperature signal from inlet temperature sensor, was adapted to control the heaters so as to achieve and maintain certain bulk temperature. Likewise, a solenoid valve connected to high pressure nitrogen was mounted on the top of the storage tank to adjust the pressure in the loop by a PID controller.

The details of flow boiling test section can be seen in

Images were taken by a NAC Memrecam HX-6E high speed camera whose maximum frames per second (fps) can reach to 210,000. In this experiment, images were taken at 5,000 fps with resolution of 1280 × 720 pixels. A stainless steel ruler was used to calibrate the length scale in this paper. The calibrated length is 0.013 mm per pixel and the maximum uncertainty of length measurement is 2 pixels. The high speed camera was placed in parallel to the test specimen surface and was located 40 mm downstream from the beginning of test specimen. A 500 W halogen lamp was fitted on the top of the test section for illuminating the test specimen surface when nucleate boiling took place.

And all the thermodynamic properties of fluid concerned in this paper were estimated by equations presented in popiel et al. [

Before the formal testing, the bulk fluid was heated to around its boiling point (95°C) for several hours to remove the dissolved gas in the fluid. And subcooled boiling heat transfer experiment was continuously running for around 40 h before generating any experimental data in order to minimize the ageing effect [_{w} are calculated as follows:

where _{1} and _{2} are the average temperature of upper and lower thermocouple rows, _{1} (3 mm) and _{2} (5 mm) are the distance from the heated surface to upper and lower thermocouple rows, respectively, as shown in

Designation | Bulk temperature [°C] | Subcooling [°C] | Bulk velocity [m/s] | Pressure [kPa] |
---|---|---|---|---|

1 | 65.0 | 35.0 | 0.25 | 100 |

2 | 65.0 | 35.0 | 0.50 | 100 |

3 | 65.0 | 35.0 | 0.75 | 100 |

4 | 70.0 | 30.0 | 0.50 | 100 |

5 | 75.0 | 25.0 | 0.50 | 100 |

6 | 65.0 | 43.0 | 0.50 | 135 |

7 | 65.0 | 49.8 | 0.50 | 170 |

The experimental errors of variables were mainly caused by the accuracy of sensors, manufacturing tolerances, the layout of thermocouples and related functions to calculate indirect variables. Before the experiment, all the thermocouples and correspondent data acquisition system were calibrated between ice and saturation temperature of water. The inlet temperature was measured by a Pt100 temperature sensor and the system pressure was measured with strain-gauge sensors having a range from 0–10 bar. The detailed measured errors for all related variables are listed in

where

Parameters | Uncertainties |
---|---|

Bulk temperature | 0.1°C |

Bulk velocity | 0.022 m/s |

Pressure | 5000 Pa |

Thermocouple location | 0.15 mm |

Thermocouple temperature | 0.1°C |

Heat flux | ≤13.2% |

Bubble diameter | 0.026 mm |

The cross-sectional area of bubble from side was assumed to be an ellipse [_{m} and surface area _{m} were calculated as follows:

where n is the number of bubbles measured. The mean Sauter bubble diameter was defined by:

The ImageJ and MATLAB software were used to process the bubble images.

The size of bubble on the heating surface under a certain experiment condition is different from place to place. As a result, appropriate sample size should be chosen to estimate the bubble size distribution and mean bubble diameter without either decreased accuracy caused by smaller sample size or increased processing cost brought by larger sample size. In this paper, experimental condition (bulk temperature = 75°C, bulk velocity = 0.5 m/s, pressure = 1 bar, heat flux = 688.5 kW/m^{2}) was chosen to estimate the bubble sample size since it has the largest range of bubble size distribution. Then this sample size was adopted in all experiments to conduct statistical analysis.

In the work of Zeitoun et al. [

where

The mean bubble diameter of sample size 2000 was used as truth value in the present work. Sample size n begins from 200 with a 200 interval till 1800. Note that the interval of two chosen frames was increased to 2 milliseconds or 10 frames so as to guarantee the statistical relevance of the mean value. The result is shown in

The influence of fluid velocity on mean bubble diameter is presented in

However, the effect of bulk velocity on bubble mean diameter seems to be reversed in high and low subcooling Jakob number region. As pointed by Zeitoun et al. [

As it can be seen from

Zeitoun et al. [

In this paper, a comparison between experimental mean bubble diameter and predicted mean bubble diameter by

Although large deviation existed when applying

In this paper, visualization experiments were conducted in horizontal rectangular channel under subcooled flow conditions. The heating surface was located at the bottom of the channel and bubble images on that surface were taken from the side view. The experimental conditions of bulk temperature, bulk velocity and pressure were 65°C to 75°C, 0.25 m/s to 0.75 m/s and 1 bar to 1.7 bar, respectively. The effects of subcooling, bulk velocity, pressure and heat flux on the mean Sauter bubble diameter was investigated. And it was found that decreased subcooling, bulk velocity, pressure and increased heat flux would increase the mean bubble diameter. Over-prediction had been found comparing experimental data in this paper and former proposed equation by Zeitoun et al. [

minimum diameter of bubble, mm

maximum diameter of bubble, mm

_{p}

liquid specific heat, J/kg K

_{s}

mean Sauter bubble diameter, mm

gravitational acceleration, m/s^{2}

mass flow rate, kg/m^{2} s

Distance between thermocouples to heated surface, mm

_{fg}

latent heat, J/kg

thermal conductivity, W/m2k

heat flux, kW/m^{2}

temperature, °C

_{m}

mean bubble surface area, mm^{2}

_{m}

average bubble volume, mm^{3}

boiling number = _{fg}

Jakob number based on liquid subcooling = _{t}_{p}_{s} _{l}_{g}_{fg}

Reynolds number

surface tension, N/m

density, kg/m^{3}

density difference, _{l}_{g}, kg/m^{3}

liquid

vapor

saturation

^{2}