Extended surfaces boiling

Forced Convection 186 Condensation 187 Boiling 187 Extended Surfaces 188... 

Performance Prediction of Low Temperature Air-cooling Extended Surface by Several Comparative Methods (3) 325 Heat Transfer to Helium II for Big Differences of Temperature between Heated Wall and Superfluid Helium Bath (5) 225 Boiling Heat Transfer to Liquid Hydrogen from Flat Surfaces (5) 254 Transient Phenomena Associated with the Pressurized Discharge of a Cryogenic Liquid From a Closed Vessel (5) 467... 

Enhancement techniques may be conveniently divided into two classes - passive methods and active techniques. Extended surfaces which require no direct application of external power fall within the passive category. Rotation (as used in the Rotex heat pump initially mentioned in Chapter 1) is an active technique, as obviously energy is needed to rotate the device. Additionally, each technique may be applicable to one or more modes of heat transfer (e.g. forced convection, boiling, condensation, etc.) and compound enhancement - the use of more than one technique on a single heat exchanger - is practised. 

Tlie fin is the most common fonn of extended surface, associated with both tubular and plate-type heat exchangers - the car radiator is a highly compact finned unit. Most heat exchangers used in gas streams have fins to improve the gas-side heat transfer. Compact units such as the plate-fin heat exchanger have fins, or secondary surfaces , between each pair of plates. Fins are used in both natural and forced convection. They are also used, when they are sometimes called ribs, to aid boiling or to provide drainage... 

If the channel head pressure is still not low enough to utilize exhaust steam instead of high-pressure steam, consider modifying the tube bundle with extended surface (fins) or nucleate boiling (Linde Hi-Flux) tubes. The increase in effective heat-transfer coefficient or surface area will permit use of lower-pressure steam. 

As noted earlier m this section branched alkanes have lower boiling points than their unbranched isomers Isomers have of course the same number of atoms and elec Irons but a molecule of a branched alkane has a smaller surface area than an unbranched one The extended shape of an unbranched alkane permits more points of contact for mtermolecular associations Compare the boiling points of pentane and its isomers... 

In boiling liquids on a submerged surface it is found that the heat transfer coefficient depends very much on the temperature difference between the hot surface and the boiling liquid. The general relation between the temperature difference and heat transfer coefficient was first presented by Nukiyama(77) who boiled water on an electrically heated wire. The results obtained have been confirmed and extended by others, and Figure 9.52 shows the data of Farber and Scorah<78). The relationship here is complex and is best considered in stages. 

Thus, the enhancement of heat transfer may be connected to the decrease in the surface tension value at low surfactant concentration. In such a system of coordinates, the effect of the surface tension on excess heat transfer (/z — /zw)/ (/ max — w) may be presented as the linear fit of the value C/Cq. On the other hand, the decrease in heat transfer at higher surfactant concentration may be related to the increased viscosity. Unfortunately, we did not find surfactant viscosity data in the other studies. However, we can assume that the effect of viscosity on heat transfer at surfactant boiling becomes negligible at low concentration of surfactant only. The surface tension of a rapidly extending interface in surfactant solution may be different from the static value, because the surfactant component cannot diffuse to the absorber layer promptly. This may result in an interfacial flow driven by the surface tension gradi-... 

Into a 2-1. round-bottomed flask fitted with a 35-cm. reflux condenser are placed 250 g. (0.74 mole) of N-grade wood rosin (Note 1), 740 ml. of 95% ethanol, and 42 ml. of hydrochloric acid (sp. gr. 1.19). A stream of carbon dioxide is passed over t lie surface of the solution by means of a glass tube which extends downward through the condenser during this reaction (Note 2). The mixture is boiled under reflux for 2 hours (Note 3). At the... 

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