Evaporator as a super heater
Distributions of temperatures along the length of the TMMHP for different working fluids, for different heat inputs, and also for different inclinations are shown from Figure 5 (a) to 12 (b). It is observed from these figures that in each case of fluid used in TMMHP, there is a temperature rise in the evaporator from T1 to T2. Annamalai A. S. et al. [7] has reasoned that “In the evaporator zone heat is supplied by an electric coil and the coil surface area density is very high in the middle of the evaporator portion and hence the temperature of the vapor in the middle of the evaporator is high”. Authors opine different from Annamalai that there should be no reason to windup the heater coil densely in the middle of the evaporator rather wrapping must be uniformly done so the produced heat flux remains constant throughout the evaporator. Based on this work and the work of Sakib [6] and Sreenivasa [4], the working fluid should be filled only equal to or less than the empty space (vapor core) of the evaporator of the heat pipe. However, a lot more space in the heat pipe is still vacant to travel during operation. Soon after the MHP goes on operation, boiling starts at the beginning of the heat pipe—part of the fluid evaporates—that leaves a significant room empty within the evaporator which is fully wrapped up by the heater coil. Therefore, when the saturated vapor advances, it continuously receives heat from that part of heater to become superheated, and then it enters the adiabatic section. That’s why we notice the temperature rise at point T2, hence this end of the evaporator act as a super heater .
[CHART] [CHART]
Figure 5 (a). Fluid temp. distribution along the TMMHPFigure 5 (b). Fluid temp. distribution along the TMMHP
[CHART] [CHART]
Figure 5 (c). Fluid temp. distribution along the TMMHPFigure 5 (d). Fluid temp. distribution along the TMMHP
[CHART] [CHART]
Figure 6 (a). Fluid temp. distribution along the TMMHPFigure 6 (b). Fluid temp. distribution along the TMMHP
[CHART] [CHART]
Figure 6 (c). Fluid temp. distribution along the TMMHPFigure 6 (d). Fluid temp. distribution along the TMMHP
[CHART] [CHART]
Figure 7 (a). Fluid temp. distribution along the TMMHPFigure 7 (b). Fluid temp. distribution along the TMMHP
[CHART] [CHART]
Figure 7 (c). Fluid temp. distribution along the TMMHPFigure 7 (d). Fluid temp. distribution along the TMMHP
[CHART] [CHART]
Figure 8 (a). Temp. distribn. along TMMHP for diff. fluidsFigure 8 (b). Temp. distribn. along TMMHP at diff. fluids
[CHART] [CHART]
Figure 9 (a). Temp. distribn. along TMMHP at diff. incln.Figure 9 (b). Temp. distribn. along TMMHP at diff. incln.
[CHART] [CHART]
Figure 10 (a). Temp. distribn. along TMMHP at diff. incln.Figure 10 (b). Temp. distribn. along TMMHP at diff. incln.
[CHART] [CHART]
Figure 11 (a). Temp. distribn. along TMMHP at diff. incln.Figure 11 (b). Temp. distribn. along TMMHP at diff. incln.
[CHART] [CHART]
Figure 12 (a). Temp. distribn. along TMMHP at diff. incln.Figure 12 (b). Temp. distribn. along TMMHP at diff. incln.
Regarding the change of temperature in the evaporator, a comparative relationship between the fluids in the TMMHP and in the SMMHP [6] is shown in Figure 13. Temperature rise for methanol is found to be higher than those of other three fluids; however, they all are polynomially developed and found to be the best fit on the order of six. It is obvious that the boiling points of the fluids play an important role in superheating—the higher the boiling point, the complexier the function is. On the other hand, in the case of SMMHP [6, 7] the trend is negative which indicates no superheating effect present in it.
[CHART]
Figure 13. Comparison of temp. diff. (T2-T1) of TMMHP of difft. fluids with the SMMHP [6] at incln. of 0o
Since the coolant flow rate remained constant throughout the experiments, the condensation of the fluids must be depended on the specific heat capacity of them, thus their value must vary. It is noticed the temperature rise of all fluids are maintaining their own tracks with an almost certain pace. Methanol has given the highest rise while ethanol the minimum. However, the lowest rise of temperature of ethanol may be explained as its earliest evaporation and speedy escaping the evaporation chamber due to pressure rise.
Figure 14 indicates that water was condensed within the smallest temperature band because of the highest specific heat (Cp). Considering this, methanol should have possessed the largest bandwidth, but it is found that ethanol has got the highest bandwidth. Thus it is proved again that the heat capacity of a liquid is not only depended on its physical property but also on its chemical property (i.e. structural bonding).
While being condensed, the internal working fluids were experiencing negative temperature gradient within the condenser, but the water is an exception with a positive gradient as shown in Figure 15. This behavior of water is due to the impact of the saturated liquid at the end of condenser.
The advancing water vapor from the adiabatic section towards the condensation port (T4) creates high pressure on the saturated liquid constantly that accelerates the liquid particles of heavy momentum to hit the other end of the heat pipe.
[CHART] [CHART]
Figure 14. Band-width of condensn. temp. (T4,16W - T4,2W) of Figure 15. Comparison of condensn . temp. (T5-T4) range of
diff.fluids for diff. heat inputs applied to TMMHP diff. fluids for increasing heat inputs to TMMHP
.
During this impact, the inherent kinetic energy of the liquid is converted to heat, hence the temperature of the liquid increases. At the turning point, such a temperature increase of the liquid benefits the capillary action of the wick to drive back condensate even faster to the evaporator. A comparison of thermal performances between the single-metal and two-metal micro heat pipe has been established in the Table 2.
The efficiency of MHP is highlighted by its heat transfer capability at a lower temperature difference. A comparison between the TMMHP and SMMHP [6] is shown in Figure 16. As it is seen, the terminal temperature difference at TMMHP is only the fourth or even less than that of at SMMHP. This has become possible because of relatively higher conductivity of silver at the condenser port that accelerates the thermodynamic cycle of the working fluid within the heat pipe.
[CHART]
Figure 16. Comparison of terminal temp. diffs. (T1 -T5) of TMMHP with SMMHP [6] at 45o