THE HEAT TRANSFER COEFFICIENTS OF BOILING LIQUIDS
LIN CHI-FANG YANG YU-CHE KOON FAN-UON (Dairen Institute of Technology)
1958, 9(2):
102-112.
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In this paper, we have investigated the influences of heating surfaces, using copper, brass, stainless steel, nickel-plated, and chromium-plated heating suffaces. We found that, brass surface gave the highest a, while chromium-plated surface the lowest. This can be explained by the fact that the contact angle between liquids and metallic surfaces are different.
The influences of heating surfaces are important only when the heat flow is comparatively low, such as q≤40000 kcal/m2.hr; while for higher heat flow, such as q≤1.5×105 kcal/m2.hr, the influences of heating .surfaces are not apparent.
With heating surfaces of the same material, we found that aged surfaces gave lower ?than fresh surfaces, but the slopes of a - f(q) relationship are the same. For scaled surfaces, ?is not only lower than both fresh and aged surfaces, the slope of a = f(q) is also flater; this is due to the heat resistence offered by the deposited scale.
For heating surfaces of the same geometrical form, the dimension is not essential for ordinary cases, but for experimental apparatus of extremely small sizes, such as single heating wires of diameter less than 0.4 mm, or small circular plate of diameter less than 10 mm, the influences of dimension are not negligible.
Finally, we have investigated the heat transfer coefficient of boiling solutions (5% - 20% NaCl solutions) The result is that for dilute solutions, the decreasing rate of a is rather high, while for concentrated solutions, the decreasing rate of ?is not so apparent.
When correlated with dimensionless groups, we found that the equation suggested by P. H. KPY>KHJIHH may be used for calculations of salt solutions, but the following corrected form is found to be more satisfactory, with error not greater than±7%,
The experiments were carried out with boiling liquids outside horizontal tubes (of diameters 25 - 60 mm) and above horizontal circular plates (of diameters 10 and 15 mm). The heat flow range of horizontal tubes is 1300-40000 kcal/m2. hr, while that of the horizontal plates is 20,000-900,000 kcal/ m2.hr.