gas liquid interface and the dimensions of the detailed structure of the heat conductive member shown in fig5 the lengths w 1 w 2 w 3 and w 4 of the sides of the tunnel sections and the diameter d p of the aperture for example when the medium is boiled using one layer of horizontal tunnels 19 a very high boiling heat transfer coefficient is attained in a case where as the vapor bubbles escape through the apertures a suitable amount of liquid enters the tunnel through the other aperatures generating no vapor bubbles and then vaporizes by way of example fig1 shows how the performance in the case of employing a panel structure composed of one layer of horizontal tunnels and openings and boiling fluorocarbon liquid fc 72 under the atmospheric pressure changes depending upon the diameters of the openings the logarithm of the heat flux q w cm 2 with reference to a projection area which does not include the increase of a substantial heat transfer area based on the provision of the panel structure is taken on the axis of ordinates while the logarithm of the difference u03b4t u00b0 c between the temperature of the heat transfer surface and the saturation temperature 56 u00b0 c of the liquid is taken on the axis of abscissas all the sample panel structures have the same tunnel dimensions 0 4 mm u00d7 0 25 mm tunnel provision pitches 0 55 mm and opening provision pitches 0 7 mm and the samples differ in only the.