Micro heat pipe

Experimental results have been published for micro tubes [1], micro channels [2], and micro heat pipe [3]. The micro scale experimental results differ from the prediction of conventional models. Some neglected phenomena must be taken into account in micro scale convection. One of them is the Knudsen number defined as the ratio of the molecular mean free path to characteristic length of the micro conduit. 

Petersen G. P., A. B. Duncan and M. H. Weichold, 1993, perimental investigation of micro heat pipes fabricated in silicon wafer, ASME J. Heat Transfer, vol. 115, pp.751-756. 

MICRO HEAT PIPE EFFECT IN POROUS MEDIA... 

The process inside a sintered powder porous body is similar to the processes inside micro heat pipes (Figure 7) with zone of evaporation and condensation. If the tested sample is placed in liquid pool, we have micro heat pipe closed type (Figure 7, a), for partially flooded porous cylinder we have the surface with micro heat pipe open type (Figure 7, b). 

Figure 7. Micro heat pipe effect in porous coating a) closed type, b) open type...

Porous covering with micro heat pipe phenomena stimulates the evaporative heat transfer near 8-10 times to compare with pool boiling heat transfer on the smooth horizontal tube. 

Micro heat pipe effect -l- two-phase forced convection in the coaxial gap (porous tube inside the glass tube) increase the heat transfer 2-2.5 times as higher as in liquid pool at low and moderate heat fluxes. 

Badran B., Gerner F., Ramada R, Henderson T., Baker K., (1997), Experimental results for low - temperature silicon micromashined micro heat pipe arrays using water and methanol as working fluids, Experimental Heat Transfer, 10,253 - 272... 

Wang Y., Ma H., Peterson G, (2001), Investigation of the temperature distribution on the radiator fins with micro heat pipes. Journal of Thermophysics and Heat Transfer, 15 42 - 49... 

Kang S., Tsai S., Ko M., (2004), Metallic micro heat pipe heat spreader fabrication. Applied Thermal Engineering, 24,299 - 309. 

Launay S., Sartre V., Lallemand M., (2002), Thermal study of water-filled micro heat pipe including heat transfer in evaporating and condensing microfilms. Proceedings of the l International Heat Transfer Conference, Grenoble, France, August 18 - 23, 6 p. 

In the applications, industrial heat exchanges are mini-and-microscale heat transfers, miniature and micro heat pipes and heat transfer issues in cryogenic catheters are presented. Nanotechnology and heat transfer including heat transfer characteristics of silicon film irradiated by pico to femtosecond lasers are also introduced and discussed. 

There are a number of different ways to classify heat pipes, but perhaps the two most important categories are the variable-conductance heat pipes (those in which the magnitude and/or direction of the heat transfer can be controlled) and micro-heat pipes (those that are so small that the mechanisms controlling their operation are significantly different from those in more conventional heat pipes). 

In 1984, Cotter [39] first introduced the concept of very small micro -heat pipes incorporated into semiconductor devices to promote more uniform temperature distribution and to improve thermal control. At that time a micro-heat pipe was defined as one so small that the mean curvature of the liquid-vapor interface is necessarily comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. Since this initial introduction, numerous investigations have been conducted on many different types of relatively small heat pipes. Many of these devices were in reality only miniaturized versions of larger, more conventional heat pipes, while others were actually significantly different in their design. 

To better understand what the term micro-heat pipe implies, Babin and Peterson [40] expressed Cotter s initial definition of a micro-heat pipe mathematically as... 

The fundamental operating principles of micro-heat pipes are essentially the same as those occurring in larger, more conventional heat pipes. Heat applied to one end of the heat pipe vaporizes the liquid in that region and forces it to move to the cooler end, where it condenses and gives up the latent heat of vaporization. This vaporization and condensation process causes the liquid-vapor interface in the liquid arteries to change continually along the pipe,... 

T. P. Cotter, Principles and Prospects of Micro Heat Pipes, Proc. 5th Int. Heat Pipe Conf, Tsukuba, Japan, pp. 328-335,1984. 

G. P. Peterson, An Overview of Micro Heat Pipe Research, Applied Mechanics Review (45/5) 175-189,1992. 

G. P. Peterson and A. K. Mallik, Transient Response Characteristics of Vapor Deposited Micro Heat Pipe Arrays, ASME J. Electronic Packaging (117/1) 82-87,1995. 

In the course of the fast growing development of microfluidic devices like Lab-on-a-Chip, microreactors, and micro heat pipes, it is essential to develop effective systems for measuring the temperature distribution in the microscale. For example, for observing the heat transfer in microchannel flows, it is a matter of particular interest to know both the temperature distribution along the wall and the temperature distributirHi in the fluid flow. Conventional methods for temperature measurements are based on thermocouples and resistance thermometers which have a minimum size of about 10 pm and which always interfere with the system to be measured. Contactless methods for measuring temperature, e.g., infrared thermography, have many... 

The heat pipe is a device that utilizes evaporation heat transfer in the evaporator and condensation heat transfer in the condenser in which the vapor flow from the evaporator to the condenser is caused by the vapor pressure difference, and the liquid flow from the cmidenser to the evaporator is produced by capillary force, gravitational force, electrostatic force, or other forces directly acting on it. A micro heat pipe is so small that the mean curvature of the liquid-vapor interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. Mathematically, the definition of micro heat pipe can be expressed as... 

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