5.3 Realization
5.3.2 Heat exchanger
A heat exchanger has been used a the temperature controller in Figure 5.3. This temperature controller has to cool the to be evaporated water to as close to 4◦. A schematic overview of this heat exchanger is shown in Figure5.8.
The heat exchanger is constructed out of three pipes, connected to each other by a POM cap at the top and a stainless steel support at the bottom. Water flows from the bottom via the support though a stainless steel tube to the evaporating geometry. Both support and tube are made of stainless steel 316 to minimize the effect of oxidation as purified water is corrosive and
CHAPTER 5. DESIGN OF THE EXPERIMENTAL SETUP
(a)
(b)
Figure 5.7: Schematic overview(a) and realization(b) of the thermocouple guidance system.
(a)
(b)
Figure 5.8: Schematic section view of the heat exchanger (a), and the position of the inlet channel (b)
should not contaminate the water by dissolved metals. In addition, the stainless steel tube has a high thermal conductance which improves the exchange of heat between the cooling water and the to be evaporated water and the stainless steel support causes for a stable and precise connection between the whole setup and the stabilizing table. A disadvantage of this that heat of the heat exchanger can leak easily away though the table due to the high conductivity of the stainless steel support . However, stability is considered more important than the heat losses as a simple calculation will show that cooling capacity is high enough.
The outer pipe, which separated the cooling water from the vacuum, the middle pipe, which separates the two streams of cooling water and the cap are made of POM. This material has good mechanical properties, which are needed to withstand the force of the pressure difference and the screwing cap on top of it. in addition, it has a very low thermal conductivity which reduces the heat losses inside the vacuum chamber.
The heat exchanger has been connected to a cooling bath which can accurately control the temperature with the use of a thermocouple placed at the inlet of the vacuum chamber. This cooling bath is connected to a pump to pump the cooling water through the heat exchanger and has a flow rate of 15 liter per minute.
A small calculation verifies that the available heat capacity is high enough to cool the to be evaporating water and to overcome heat losses.
The energy supplied by the cooling bath.
Qin= (Tamb− Tevap)mcoolingbath˙ cp (5.1) Where Tamb is the romm temperature in the lab, Tevap is the temperature to which the ’to be evaporated water’ as to cool down and cp is the specific heat.
The energy outflow:
Qout = (Tamb− Tevap)mevap˙ cp (5.2) The larges heat loss is the heat conducted from the inlet of the heat exchanger to the stabilizing
CHAPTER 5. DESIGN OF THE EXPERIMENTAL SETUP
Figure 5.9: The conduction of heat through the stainless steel flange to the stabilizing table has been considered to be the highest heat lost. The value A and L are defined in this Figure.
table. It is assumed that the heatlosses in the vacuum part of the heat exchanger are negligible.
Qloss= κststA
L (Tevap− Tamb) (5.3)
Where A is the cross section area of the stainless steel support, L is the distance between the cooling water and the stabalising table and κstslis the thermal conductivity coefficient of stainless steel (See Figure5.9).
φ = Qin
Qout+ Qloss (5.4)
For mass flow of 100 µl per hour and an Tevapof 4 degree Celsius (which are typical conditions in the experiments of Fang et al.[13]), φ ≈ 103, so the capacity of the cooling bath is high enough to overcome heat losses and to cool the to be evaporated water.
Results
6.1 Results
An attempt has been made the conduct the experiments for which the experimental setup has been designed for. Which is the measurement of the temperature profile and mass flux of an evaporating droplet at pressures range from 200 to 1000P a. However, once the pressure in the chamber becomes lower than 1000P a, bubbles start to form inside the syringe and in the channel between the syringe and the droplet geometry (see Figure6.1a for an example of the formation of a bubble). Due to the sudden expansion of these bubbles, water inside the droplet geometry was blown away inside the vacuum chamber. The cause of these bubbles did not become clear during the experiments. As they kept occurring even after two hours of degasification and they did not seem to origin from a specific point in the setup. These bubbles made it impossible to conduct the original planned experiments.
Although the planned experiments could not have been conducted, a few adjustments to the original setup made it possible to conduct a simplified measurement. The syringe had been removed and the supplier channel for the droplet has been connected directly to the degasification vessel. Although the bubbles kept occurring inside the channel, they did not disturb the droplet inside the geometry, as they now prefer to flow in the opposite direction towards the degasification channel. In order to keep the height of the droplet during evaporation constant, the degasification vessel had been placed at the same height of the droplet. With the assumption that the water level in the degasification channel does not change significantly, the height of the droplet is in this way controlled by hydro static pressure.
With these adjustments the device was able to do a simple measurement of the temperature profile near the interface at about 780 Pa. At this pressure the pressure fluctuations due to the gas bubbles was about 30P a. At lower pressure these fluctuations became as high as 100P a. Which was unacceptable for an accurate measurement at these pressures.
Before and after the start of the experiments, the thermocouple was calibrated in the same way as the setup described in Chapter 4. Which was in a bath of boiling and ice water. In which an offset and a proportionality constant have been determined at the two measuring points. The measured temperature did not change significantly and was not affected by the experiments.
Before the start of the experiment, water was placed in the degasification channel and degassed for about an hour. Meanwhile the cooling device and pump were turned on to cool the evapor-ating liquid. After this hour, the measured temperature of the heat exchanger, and the small thermocouple above the interface (with fixed position) did not change significantly.
The temperature profile of the interface was measured between 4mm above the interface and 1, 5mm underneath it. With steps of 5µm near the interface. At the interface, it had been observed that the thermocouple has to overcome the surface tension before it is fully immersed in the liquid.
CHAPTER 6. RESULTS
(a) (b)
Figure 6.1: An example of the formation of a bubble inside the syringe(a) and the adjusted setup where the syringe is removed and the degasification vessel has been lowered to the droplet height(b).
In order to overcome this problem, the thermocouple was first fully immersed in the liquid and before the measurement of the liquid temperature close to the interface.
The measured temperature profile as function of the overall droplet radius has been plotted in Figure6.2a. In this figure, one can see that the interface has a lower temperature than the far field temperature of the vapor and the liquid. This would imply that heat is conducted towards the interface. Which makes sense as this heat is required for evaporation. In addition, the temperature gradient in the liquid phase is lower than the temperature in the vapor phase. which indicates a higher thermal conductivity of the liquid than of the water. Due to the error in the temperature measurement, caused by the bubbles, it was impossible to measure a temperature jump at the interface.
In order to estimate the magnitude of the pressure fluctuations and the effect of this on the temperature measurement, the position of the thermocouple was fixed for a few minutes after the measurement. The measured pressure, and temperature over this time have been plotted in Figure 6.2b. In this figure, the sudden nucleation and outburst of the bubbles can be identified as high peak in both pressure and temperature.
Position [m] ×10-3
Figure 6.2: The measured temperature at the interface (a). An example of the pressure and temperature fluctuations caused by the occurring bubbles during an experiment(b).
Figure 6.3: The approximated geometry of droplet by the spherical model.