HEAT PIPES

1)      A heat pipe is a simple device with no moving parts that can transfer large quantities of heat over fairly large distances essentially at a constant temperature without requiring any power input. 2)      It is composed of three sections: a)      Evaporator section at one end, where heat is absorbed and the fluid is vaporized b)      Condenser section at…

Read More

THE PRESENCE OF NON-CONDENSABLE GASES IN CONDENSERS

1)      Most condensers used in steam power plants operate at pressures well below the atmospheric pressure to maximize cycle thermal efficiency, and operating at such low pressures raises the possibility of air (a non-condensable gas) leaking into the condensers. 2)      Even small amounts of a non-condensable gas in the vapor cause significant drops in heat transfer coefficient…

Read More

DROPWISE CONDENSATION

1)      Dropwise condensation, characterized by countless droplets of varying diameters on the condensing surface instead of a continuous liquid film, is one of the most effective mechanisms of heat transfer, and extremely large heat transfer coefficients can be achieved with this mechanism 2)      In dropwise condensation, the small droplets that form at the nucleation sites on the surface grow…

Read More

FILM CONDENSATION

1)       increases in the flow direction x because of continued condensation at the liquid–vapor interface. a)      Heat in the amount hfg (the latent heat of vaporization) is released during condensation and is transferred through the film to the plate surface at temperature Ts 2)      Heat Transfer Correlations for Film Condensation a)     Vertical Plates i)        Assumptions (1)   Both the plate and the vapor are maintained at constant…

Read More

CONDENSATION HEAT TRANSFER

1)      Condensation occurs when the temperature of a vapor is reduced below its saturation temperature a)      It can occur at solid surface as well as free surface of liquid or the gas. b)      Two distinct forms of condensation are observed: i)        Film condensation: The condensate wets the surface and forms a liquid film on the surface that slides down under…

Read More

FLOW BOILING

1)      In flow boiling, the fluid is forced to move by an external source such as a pump as it undergoes a phase-change process. The boiling in this case exhibits the combined effects of convection and pool boiling 2)      The flow boiling is also classified as either external and internal flow boiling a)      External flow boiling over a plate…

Read More

POOL BOILING

1)      Steps involved a)      Early stages of boiling, some bubbles stick to the surface of the pan caused by the release of air molecules dissolved in liquid water and should not be confused with vapor bubbles b)      At temperature rise chunks of liquid water rolling up and down as a result of natural convection currents, followed by the…

Read More

BOILING HEAT TRANSFER

1)      Boiling is a liquid-to-vapor phase change process just like evaporation, but there are significant differences between the two a)      Evaporation occurs at the liquid–vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature. b)      Boiling, on the other hand, occurs at the solid–liquid interface when a liquid is…

Read More

THE EFFECTIVENESS–NTU METHOD

1)      This method is based on a dimensionless parameter called the heat transfer effectiveness 2)      To determine the maximum possible heat transfer rate in a heat exchanger, we first recognize that the maximum temperature difference 3)      The heat transfer in a heat exchanger will reach its maximum value when a)      The cold fluid is heated to the inlet temperature…

Read More

THE LOG MEAN TEMPERATURE DIFFERENCE METHOD

)       a)      The expressions in parallel-flow and counter-flow heat exchangers. b)      For specified inlet and outlet temperatures, the log mean temperature difference for a counter-flow heat exchanger is always greater than that for a parallel-flow heat exchanger. That is,  and thus a smaller surface area is needed to achieve a specified heat transfer rate in a counter-flow heat exchanger….

Read More