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 .
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Figure 5 (a). Fluid temp. distribution along the TMMHPFigure 5 (b). Fluid temp. distribution along the TMMHP
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Figure 5 (c). Fluid temp. distribution along the TMMHPFigure 5 (d). Fluid temp. distribution along the TMMHP
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Figure 6 (a). Fluid temp. distribution along the TMMHPFigure 6 (b). Fluid temp. distribution along the TMMHP
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Figure 6 (c). Fluid temp. distribution along the TMMHPFigure 6 (d). Fluid temp. distribution along the TMMHP
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Figure 7 (a). Fluid temp. distribution along the TMMHPFigure 7 (b). Fluid temp. distribution along the TMMHP
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Figure 7 (c). Fluid temp. distribution along the TMMHPFigure 7 (d). Fluid temp. distribution along the TMMHP
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Figure 8 (a). Temp. distribn. along TMMHP for diff. fluidsFigure 8 (b). Temp. distribn. along TMMHP at diff. fluids
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Figure 9 (a). Temp. distribn. along TMMHP at diff. incln.Figure 9 (b). Temp. distribn. along TMMHP at diff. incln.
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Figure 10 (a). Temp. distribn. along TMMHP at diff. incln.Figure 10 (b). Temp. distribn. along TMMHP at diff. incln.
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Figure 11 (a). Temp. distribn. along TMMHP at diff. incln.Figure 11 (b). Temp. distribn. along TMMHP at diff. incln.
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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.
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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.
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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.
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Figure 16. Comparison of terminal temp. diffs.
(T1 -T5) of TMMHP with SMMHP [6] at
45o