Heating effects, droplet transient

Transient Heating of Droplets When a cold liquid fuel droplet is injected into a hot stream or ignited by some other source, it must be heated to its steady-state temperature Ts derived in the last section. Since the heat-up time can influence the V/2 law, particularly for high-boiling-point fuels, it is of interest to examine the effect of the droplet heating mode on the main bulk combustion characteristic—the burning time. 

In the next section some of the important time scales are identified and transient droplet heating effects during the spherically symmetric, single-component droplet vaporization are reviewed. Spherically symmetric, multicomponent droplet vaporization and droplet vaporization with nonradial convection are discussed in later sections. 

The d -law assumes a constant Tg. However, in many practical situations the temperature of the droplet when introduced into the evaporator is far below this final, equilibrium value. Hence an initial transient heating period exists during which y, and Tf all increase whereas H decreases. Furthermore it can be estimated also that the sensible heat required to heat the droplet is of the same order as the latent heat of vaporization. Hence droplet transient heating effects on the bulk vaporization characteristics are expected to be significant. Two such models, representing extreme rates of internal heating, will be discussed. 

Fig. 3 Spotting tools for non-contact printing a Bubble ink-jet A heating coil locally heats the loaded sample, resulting in a changed viscosity and expansion of fluids. The generated droplet can be easily expelled from delivery nozzles, b Microsolenoid A microsolenoid valve, fitted with the ink-jet nozzle is actuated by an electric pulse transiently opening the channel and dispenses a defined volume of the pressurized sample, c Piezo ink-jet A piezoelectric transducer that is fitted around a flexible capillary confers the piezoelectric effect based on deformation of a ceramic crystal by an electric pulse. An electric pulse to the transducer generates a transient pressure wave inside the capillary, resulting in expulsion of a small volume of sample...

Transient cooling rates were measured to study the effect of adding droplets. The cooling time is defined as the time to cool the test plate from 500 C to 35°C. These measurements were made with both the single-phase and two-phase flows. A shutter was used to close the nozzle exit to avoid any flow impingement on tiie test plate. The test plate was heated to 500°C with a constant power input of 310 Watts. The same power input was used for each experiment to msure that, once the plate reached the desired temperature, the insu tion and the heater reached the same conditions for every test Once the piste was heated up to 500°C, the shutter was opened and the heater was turned off to start the cooling process. 

An experimental stu% was performed to determine fee effect of small water droplets incorpmated into a single-phase stagnation-point flow on heat transfer. Steady state and transient cooling heat transfo erqieriments were carried out to analyze fee characteristics of single-phase and two-phase flows. PIV measurements were made for different air velocities to determine the characteristics of the flow. Water droplets size distribution was also measured. The following conclusions can be made fiom this study ... 

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