Liquid Nitrogen Cooling in an Electronic Equipment under low pressure

1. LN2 Spray Cooling In Electronic Equipment Under Low Pressure
AWAIS ARSHAD
SIBTAIN DOGAR
USAMA ARIF
ME-09(A)

2. Introduction
 Sensitivity Of Electronic Equipment On Temperature.
 Ambient Conditions & Heat Dissipation Methods
 The OPTIMUM Method (LN2Spray Cooling)

3. Explanation for Spray Cooling Method
 “Spray cooling is a technology that combines impact
convection heat transfer and phase transition heat
and is characterized by high heat dissipation rate,
of heat removal and stable cooling performance.”
Prior to 2006, Spray cooling mechanisms were not
conclusively identified due to the lack of advanced techniques
e.g., very high-speed videos, to measure spray parameters.

4. Theories that Explain the Mechanism
Two theories have been proposed to explain the mechanism of the
spray cooling:
 Thin Film Evaporation & Convection:
The thin film evaporation & convection theory indicates that
sprayed droplets produce a thin liquid film on the surface through
which the heat is conducted and this is further enhanced by the
droplet impact.
 Secondary Nucleation:
The secondary nucleation theory indicates that sprayed
droplets entrain vapor/gas into the liquid film, accelerating

5. Challenges Encountered & Possible Solution
 One of the issue related with spray cooling is low atmospheric
pressure <5530 Pa.
 That’s why, Flash evaporating of LN2 occurs at such low ambient
pressures.
 Consequently. Pressure has to be risen to 1 kPa inside the cabin
but increase in weight render less speed of aircraft.

6. Materials & Methods
 Natural Convection Under Low Pressure
The temperature for operating of electronic equipment is usually in the range
−40 °C to 50 °C & can be determined by:
Tw=T + q/hA.
At low pressures,the heat removal is mainly by natural convection.The Nusselt’s
number takes the following form
Nu = C (Gr Pr)n

7. Flow Rate C n
Laminar 0.48 1/4
Transition .0445 0.37
Turbulent 0.10 1/3
The values for C and n were taken from a chart by “Medigas Co. Ltd”
for isothermal flat plate and acrylic heaters as conventional
convection process.
Their Gr*Pr number was >109 which indicates that the flow was
turbulent.
Courtesy : Medigas Co. Ltd

8. Materials & Methods
 Thermal Control Strategy:
o Upper & Lower Temperature as well as Pressure
are to be determined.
o Heat & Mass transfer between the droplet and
surroundings are to be determined and then time
step of iteration is to be calculated.
o Transient Temperature & Pressure changes with
time during spray are to be obtained.

9. Materials & Methods
o Thermal Management:
 The temperature sensor will monitor the
temperature inside the Electronic Equipment
If the temperature is higher than the upper limit,
then the temperature sensor will turn on the
sprays to reduce the temperature of the cabin.
 Else if the temperature is below the lower limit,
then the temperature sensor will turn off the
sprays to save LN2.
 The order of the loop is ① → ② → ③ → ①

10. Materials & Methods
o Pressure Control:
 The pressure sensor will monitor the pressure of the
Electronic Equipment Cabin. If the pressure is higher
than the upper limit, then the pressure sensor will
on the vent valve to deflate the gas.
 Else if the pressure is below the lower limit, then the
pressure sensor will turn off the vent valve to collect
gas.
 The order of the loop is ④ → ⑤ → ⑥ → ④
 In addition, the dashed lines ⑦ and ⑧ indicate that
the processes of deflation and LN2 spray could cause
indirect effects on the temperature and pressure
control processes.

11. Observations & Discussion:
Consider the figure.
o The curves in (a) show the variation of LN2 droplet
lifetime with increasing diameter at P=100kPa, 50kPa,
10kPa, 5kPa and the ambient temperature is fixed at
300K.
o Correspondingly, the results in (b) are obtained at
T=350K, 320K, 280K, 250K and the ambient pressure
is fixed at 10kPa.
o It is observed that the ambient pressure has little
effect on droplet lifetime especially for the droplets
with diameter less than 100μm. While the droplet
lifetime is significantly affected by the ambient
temperature especially for the droplets with diameter
larger than 50μm.
o The results indicate that droplet with larger diameter
tends to have a relatively long lifetime especially

12. Observations & Discussion:
Consider the figure.
o It is seen that at qm=1.5×10−3kg/s, the gas
temperature rises as the sprayed LN2
cannot completely absorb the heat
generated by the electronic devices.
o When the mass flow rate of LN2 increases
to qm=1.7×10−3kg/s, the temperature
reduces rapidly.
o The pressure performs a linearly
increasing and decreasing distribution
during spray cooling.

13. Observations & Discussion:
Consider the figure.
o When the mass flow increases to qm=1.9×10−3kg/s and
qm=2.5×10−3kg/s, the gas temperature of the EEC reaches to
the lower limit rapidly.
o Furthermore, the switching frequency of the spray nozzles also
increases from 3 times to 11 times.
o This is because the heating flux is relatively high while the mass
of the gas is small due to the low pressure, which leads to the
gas temperature rising to the upper limit rapidly.
o In addition, pressure fluctuations are observed during the
switching of the spray nozzles. It is found that the larger the
mass flow, the greater the pressure fluctuation. However, the
pressure fluctuations seem to have little effect on the variation
of the gas temperature.

14. Liquid nitrogen vaporization.

15. Continuous and Intermittent cooling
• At qm=1.7x10^-3 kg/s the
temperature drops from 350K to
300K in about 290s.
• The concentration of N2 increases
and the cabin is full at the end of
the spray
• In intermittent spray cooling, the
mass flow rates are between 1.9 to
2.5 m kg/s.

16. Conclusions
 The predictions indicate that droplet with larger diameter tends to
have a relatively long lifetime, but a higher ambient temperature
will reduce the lifetime of the droplet. For liquid nitrogen droplet,
flash evaporation under low pressure seems to have little effect on
droplet lifetime.
 The spraying modes like continuous spray and intermittent spray
can be obtained by adjusting the mass flow rate of liquid nitrogen.
For the intermittent spray, the switching frequency of the spray
nozzles increases with increasing mass flow rate of LN2.
 The switching frequency of the spray nozzles is influenced greatly
by the limits of the temperature control. Reducing the lower limit
of temperature control will produce a low frequency of the spray
nozzles switching. In addition, increasing the upper limit of

17. Thank You For Your Attention!
Any Questions Please?