JPH0124996B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to a boiling heat transfer surface that has good boiling characteristics in a low heat flux region and prevents burnout in a high heat flux region, and further has good boiling characteristics in a low heat flux region. The present invention relates to a boiling heat transfer surface of a heat pipe having a heat receiving portion that prevents flattening in a high heat flux region.

Various energy-saving devices have been developed as energy-saving measures, and among them, heat pipes are being used in large numbers in exhaust heat recovery systems and solar heat utilization devices.

In particular, thermosyphon heat pipes have a simple structure and low cost, so they have a wide range of applications and are the most advanced in practical use.

However, the thermosyphon heat pipes currently in use have a smooth tube inner surface, and therefore have poor boiling characteristics, particularly in a low heat flux region.

In addition, in order to improve the boiling characteristics, research is being conducted on processing the inner surface of the tube to create a high-performance boiling heat transfer surface, but processing the inner surface of the tube is difficult due to many processing techniques and costs. It was hot.

On the other hand, the present applicant has proposed a method in which a porous membrane or a coil spring is provided on the inner surface of the heat receiving section of the heat pipe to improve boiling characteristics.

Although the above-mentioned existing proposals have improved boiling characteristics in a low heat flux region, they also have good boiling characteristics in a high heat flux region, resulting in flooding.

That is, in the high heat flux region, a large amount of vapor is generated, so that the upward flow of vapor obstructs the flow of the medium condensed in the heat radiation section.

Furthermore, on a boiling heat transfer surface, when the heat flux increases, steam covers the heat transfer surface, causing a burnout phenomenon.

If such a flattening or burnout is exceeded while the heat transfer surface superheats at a small degree, the operating characteristics as a heat pipe or a boiling heat surface deteriorate, resulting in unsteady and unstable behavior.

Therefore, the purpose of the present invention is to maintain a system in which the heat flux does not decrease even if the degree of superheating is small in the low heat flux region, and the boiling characteristics shift sideways by that much even if the degree of superheating increases in the high heat flux region. The present invention provides a boiling heat transfer surface and a heat pipe having the boiling heat transfer surface.

This will be explained below using figures.

FIG. 1 is a sectional view of a heat pipe showing an embodiment of the present invention.

The heat pipe 1 is a thermosyphon type heat pipe, and is made of stainless steel or other metal. The upper end of the heat pipe 1 serves as a heat radiating section 2, and the lower end serves as a heat receiving section 3. A working medium such as water or Freon R-113 is sealed inside the heat pipe 1.

A membrane body 4 made of synthetic resin fibers is provided on the inner surface of the tube of the heat receiving part 3. The membrane 4 has poor thermal conductivity and is porous with many spaces inside. As the material of the membrane body 4, in addition to synthetic resin, non-metallic materials such as ceramics are used. A holder 5 made of a metallic coil spring is provided inside the membrane 4 to press and hold the membrane 4 against the inner surface of the tube.

In this example, the pitch of the coil spring is
The wire diameter is 1.2 mm. Therefore, the gap between the lines is 0.2 mm. In order to explain the present invention, the results of a comparative experiment shown in FIG. 2 will be explained.

FIG. 2 is a boiling curve with heat flux (w/cm ² ) on the vertical axis and superheat degree ΔT (oK) on the horizontal axis.

For comparison, boiling curve A of a heat pipe with a smooth surface inside the tube and a coil spring with a wire diameter of 1 mm are used at a pitch of 5 mm.
The boiling curve B is shown when it is wound and provided. The boiling curve of the present invention is C.

As can be seen from Figure 2, in the boiling curve A of the smooth surface inside the tube, the heat flux is extremely low in the region where ΔT is small, and therefore the boiling characteristics in the low heat flux region of the membrane body and holder of the present invention are The excellence of is evident.

Also, the effect of the pitch of the coil spring is almost nonexistent in the region where ΔT is small, that is, in the low heat flux region,
This is clearly seen in regions where ΔT is large, that is, in high heat flux regions. In other words, boiling curve B with a pitch of 5 mm
In this case, the heat flux becomes high in a region where ΔT is large, causing a flattening phenomenon or a burnout phenomenon X. On the other hand, in the boiling curve C of the present invention with a pitch of 1.2 mm, the boiling characteristics shift sideways,
Even if ΔT increases, the heat flux does not increase by that amount. Therefore, it is possible to obtain stable operating characteristics over a wide temperature range without causing fluctuation.

The action of the above 1.2mm pitch seems to be as follows.

Since the pitch is small, the entire membrane is pressed by the coil spring, and the area where the liquid comes into direct contact with the boiling surface is small. Moreover,
Since the membrane body is made of synthetic resin, heat conduction is poor, and as a result, when the heat flow is increased, the temperature drop in the membrane body becomes large.

In addition, the coil spring holding the membrane acts as a heat absorber by dissipating heat, and is thought to prevent the degree of superheating from increasing. Furthermore, even if flattening or burnout attempts to occur, the boiling surface is subdivided by the coil spring, so only small bubbles are formed, making burnout, which is an unstable phenomenon at the gas-liquid interface, less likely to occur. . On the other hand, during the delay in the rise in the degree of superheating, heat flows out to the surrounding stable nucleate boiling surface or coil spring, making it even more difficult for burnout to occur.

As described above, in the present invention, the pitch of the holder for the membrane body made of a coil spring is made small, and the gap between the wires is made small (about 0.2 mm) to prevent floating.

Although the present invention is described as a heat pipe, it is obvious from the purpose of the present invention that the principle of the present invention can be applied to the outer peripheral surface of a tube and all other boiling heat transfer surfaces. .

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the present invention, and FIG.
The figure is a comparison diagram explaining the present invention. 1: heat pipe, 3: heat receiving part, 4: membrane body,
5: Holder.

Claims (1)

[Claims] 1. A non-metallic porous membrane is provided on the heat transfer surface, a metal wire is provided on the porous membrane, and the gap between the wires is
A boiling heat transfer surface characterized by a small gap of approximately 0.2 mm. 2. A heat pipe in which a non-metallic porous membrane is provided on the inner surface of the heat receiving part of the heat pipe, and the porous membrane is pressed against the inner surface of the tube by a coiled metal holder, A heat pipe characterized in that the pitch of the metal holder wound into a coil is slightly larger than the wire diameter, and a small gap of about 0.2 mm is provided between the wires. Boiling heat transfer surface.