Research contents

2.1.1 Technique for coincident measurement of surface temperature and heat flux

(1) Background
Compared to single phase heat transfer, nucleate boiling has extremely high heat transfer coefficient and is wildly used in heat exchanger, boiler, BWR, etc. So far, nucleate boiling has been researched a lot and is still of great importance.

Nucleate boiling is a complicate phenomenon that is consisted by bubble generation, growth up and departure. The heat transfer mechanism in the process is quite a complicate one. Heat transfer coefficient changes 4 dimensional if time axis is included. Conventional evaluation of the heat transfer coefficient uses a gross value averaged both spatially and timely. This kind of correlations is far from satisfied because they are restricted to its adaptation range.

In recent years, another kind of approach has been tried with considering the heat transfer mechanisms in a bubble cycle. Heat transfer coefficient is calculated theoretically or semi-theoretically basing on the proposed heat transfer mechanisms. For the heat transferred by evaporation underneath a bubble, there are theory that evaporation proceeds averagely in micro layer underneath the bubble and theory that evaporation concentrates in wedge shape area of micro region.

(2) Objectives
In this research, we tried to develop a measuring system for surface heat flux and surface temperature with high resolutions both spatially and timely. With using the developed system, we planned to get detailed boiling data to check the existing evaporation heat transfer theories in boiling phenomenon.

(3) Development of Measuring System
The system is consisted by two parts: (1) inner block temperatures were measured by special T-type micro thermocouples, which were located at two layers inside a heating block; (2) with the measured inner block temperatures, inverse heat conduction problem was solved to get surface heat flux and temperatures distribution.

1) Inner Block Temperature Measurements
　To measure inner block temperatures with high resolution, in this research, we developed special T-type micro thermocouples. Multi channels of temperatures were measured with using a common positive pole. The principle is shown in Fig. 2.3.1.1. In the figure, multi-points of temperature at points 1, 2, 3, 4 and 5 were measured. For each thermocouple, it had a constantan wire working as negative pole of a T-type thermocouple.

The thermocouples share a common positive pole made of copper. For each measuring point, the measured output voltages V1, V2, V3, V4, and V5 generate only on the measuring points. No extra voltages are generated along the common positive pole because of the law of homogeneous material, that a thermoelectric current cannot be sustained in a circuit of a single homogeneous material.

In this research, we used newest sputtering technology to form the copper layer working as the common positive pole. As a result, the first line of thermo sensors locates at a depth of 3.1mm and the second line locates at a depth of 4.905mm from the upper surface of the block, which serves as the boiling surface.

Fig. 2.3.1.1 Measuring principle for the special T-type thermometers with a common positive pole

2) Inverse heat conduction problem solutions
Inverse Heat Conduction Problem (IHCP) solution supplies us a method to achieve surface temperature and surface heat flux from temperature distributions inside heating block. For two-dimensional IHCP on cylindrical coordinates, if the temperatures at two layers inside heating block are measured, the surface heat flux and temperature can be predicted. Recent Woodfield et al (2006a, 2006b) IHCP solution was adopted in this research.