JPH03152343A - Boiling evaporation method and its device - Google Patents

Abstract

PURPOSE:To improve a boiling heat transfer coefficient by a method wherein some air bubbles are supplied to a part near a heat transfer surface from within its inside part or from its outside part without any relation with an applied voltage. CONSTITUTION:As thermal medium 2 is over-heated through a heat transfer surface 3 by over-heating the heat transfer surface 3 with a sheath heater 4, some bubbles are generated from an outer circumferential edge of the heat transfer edge, i.e., a part 8 having a poor heat transfer characteristic and a local high heat flux even if the bubbles are not generated from the upper surface of the heat transfer surface 3. Under this state, if a high voltage is applied between the heat transfer surface 3 and a mesh electrode 6, some bubbles are sucked into a portion between the heat transfer surface 3 and the mesh electrode 6. As the applied voltage is increased, the bubbles are restricted to the heat transfer surface 3 under an electrical induction floating force, the bubbles move severely over it. Diameter of each of the bubbles is reduced and at the same time an ascending speed of each of the bubbles is also increased. Concurrently, new bubble forming points are generated in sequence and the bubbles from these new bubbles may generate a similar phenomenon, resulting in that the heat transfer surface 3 is covered by the bubbles. With such an arrangement, a boiling heat transfer coefficient is improved.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a boiling evaporation method and apparatus.

(Prior art and its section B) Mixture of Freon R113 and ethanol and Freon R11
A heating medium consisting of a mixture of
A heating medium made by mixing a conductor with a high boiling point is smaller than 7JJ for foaming during boiling (the time it takes for bubbles to grow and separate from the heat transfer surface until the next bubble is generated). It has a charge relaxation time (the ratio between the dielectric constant and the electrical conductivity of the heating medium).

The heating medium is heated via the heating surface while a high voltage is applied between the electrode placed in the heating medium and the heating surface,
It is known that when this heat medium is allowed to boil in its boiling region, boiling may be suppressed and heat transfer may be promoted, or boiling may be accelerated and heat transfer may be promoted. There is.

It is known that the rate of promotion of heat transfer is overwhelmingly large in the latter case, and the degree of superheating of the heat transfer surface is reduced to 1750 or less.

However, until now, little was known about the mechanisms of changes in the foaming phenomenon and the phenomenon of promoting heat transfer.

(Means for Solving the Problems) The present invention aims to clarify the mechanism of the heat transfer promotion phenomenon and dramatically improve the boiling heat transfer coefficient,
The gist of the first invention is that a high voltage is applied between an electrode arranged on a heating medium cl+4 having a charge relaxation time smaller than the bubbling period during boiling and a grounded heat transfer surface. In the boiling water evaporation method of evaporating boiling food by heating the heat medium through the heat transfer surface, air bubbles are supplied near the heat transfer surface from inside or outside the heat transfer surface regardless of the applied Tl voltage. The second gist of the invention is a boiling R77 foaming method characterized by the following: 1 placed in a heating medium having a charge relaxation time smaller than the foaming period during boiling; a grounded heat transfer surface for heating; a means for applying a high voltage between the electrode and the heat transfer surface; and a means for applying a high voltage between the electrode and the heat transfer surface; A boiling R evaporation apparatus is characterized in that it is equipped with means for supplying.

Air bubbles can be supplied by high heat flux areas formed on the heat transfer surface.

The heat transfer medium can be supplied to the heat transfer surface made of a porous material.

(Function) When a heat medium is heated through the heat transfer surface while a high voltage is applied between the electrode and the heat transfer surface, the bubbles supplied to the heat transfer surface become a so-called ignition source and promote foaming. and promotes boiling heat transfer.

(Example) FIG. 1 shows an example of an actual IJ4 device.

Inside the container 1 is a heat medium 2 made of a mixture of Freon R113 and ethanol or a mixture of Freon R11 and ethanol.
is accommodated. A disk-shaped heat transfer surface 3 is provided at the bottom of the container 1, and this heat transfer surface 3 connects a sheath heater 4 to a power source 5.
It is designed to be heated by applying electricity to it. A mesoche electrode 6 is disposed in the heat medium 2 so as to be parallel to the heat transfer surface 3 by exposing a certain distance M (for example, 5 as). The heat transfer surface 3 is grounded, and the electrode 6 is connected to the positive electrode of a DC high voltage power source 7, so that a high voltage can be applied between the heat transfer surface 3 and the mesh electrode 6.

When the heat transfer surface 3 is heated by the sheath heater 4 to heat the heat medium 2 through the heat transfer surface 3, even if no foaming occurs from the upper surface of the heat transfer surface 3, the outer periphery of the heat transfer surface 3, i.e., Bubbles are generated from portions 8 where heat conduction is poor and heat flux is locally high. When a high voltage is applied between the heat transfer surface 3 and the mesh electrode 6 in this state, air bubbles are drawn between the heat transfer surface 3 and the electrode 60. When the applied voltage exceeds 10 V,
The bubbles are held down by the dielectrophoretic force on the heat transfer surface 3 and move around violently thereon. When the applied voltage exceeds 20 KV, the diameter of the bubbles becomes smaller and the rising speed of the bubbles becomes faster. As the movement of the bubbles on the heat transfer surface 3 becomes more intense, new bubble points are created one after another, and the bubbles from these new bubble points cause the same phenomenon as described above, so the heat transfer surface 3 is covered with bubbles. After that, these bubbles are swept away from the heat transfer surface 3 and the number of bubbles on the heat transfer surface 3 decreases, but the remaining bubbles move violently on the heat transfer surface 3 again and repeat the above behavior.

As a result, as shown in Figure 2, the degree of superheating at the same heat flux decreases as the applied voltage increases, and as shown in Figure 3, the heat transfer enhancement rate at the same heat flux decreases as the applied voltage increases. increases, and the heat transfer acceleration rate reaches up to about 50 times.

When the heat transfer surface 3 is integrated with the container 1, or when the container l constitutes the heat transfer surface 3, the heat transfer surface 30 may be locally heated by forming irregularities on the surface of the heat transfer surface 30 or by attaching a heat insulating material to the heat transfer surface 30. It is possible to form areas with high heat flux. . The heat transfer surface 3 may be made of a porous material, and bubbles as sparks can be supplied onto the heat transfer surface by jetting out the steam of the heat medium from numerous pores of the porous material.

1 As shown in Fig. 4, an insulating spacer 9 is placed around the heat exchanger tube I.
When a large number of linear electrodes IO are arranged through the heat exchanger tube I, the part of the heat exchanger tube I that faces the spacer 9 forms a locally high heat flux area, and the area between the heat exchanger tube I and the spacer 9 Air bubbles that act as ignition sources are generated from small gaps in the air.

(Effect of the invention) In the present invention, foaming may be stopped when a high voltage is applied to the vicinity of the heat transfer surface from inside or outside, that is, when a high voltage is applied to the boiling armpit of the heating medium. By supplying bubbles that serve as ignition sources, it is possible to promote foaming and dramatically improve the boiling heat transfer coefficient without being affected by any negative effects.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view showing an example of an experimental apparatus that can be used for experiments of the present invention, Figure 2 is a diagram showing the relationship between superheating degree, applied voltage, and heat flux, and Figure 3 is electrical conduction. FIG. 4 is a diagram showing the relationship between heat transfer rate, applied voltage, and heat transfer acceleration rate, and FIG. 4 shows an example of a heat transfer tube that can be used to implement the present invention. b) is a cross-sectional view. Electrode 6, Heat transfer surface 3, High voltage application means 7, Air Figure 1 Figure 3 Figure 2 Factor Electrification 5RWi l7e (+7Q, ) x 1σ8 Excess+#) degree ΔTsat (K)

Claims (4)

[Claims] (1) A high voltage is applied between the grounded heat transfer surface and an electrode placed in a heat medium having a charge relaxation time smaller than the bubbling period during boiling, and the heat transfer surface is A boiling evaporation method for boiling and evaporating the heat medium by heating the heat medium, characterized in that bubbles are supplied near the heat transfer surface from inside or outside the heat transfer surface regardless of an applied voltage. (2) An electrode disposed in a heat medium having a charge relaxation time smaller than the bubbling period during boiling, a grounded heat transfer surface for heating the heat medium, and the electrode and the heat transfer surface. 1. A boiling evaporation device comprising means for applying a high voltage between the heat transfer surface and means for supplying bubbles from inside or outside of the heat transfer surface regardless of the applied voltage. (3) Claim (2) characterized in that the means for supplying the bubbles is a high heat flux portion formed on the heat transfer surface.
) Boiling evaporation device described. (4) Boiling evaporation according to claim (2), wherein the heat transfer surface is made of a porous material, and the means for supplying the bubbles is a means for supplying vapor of the heat medium to the porous material. Device.