Chapter 3. Design of Experimental Set Up

Chapter 3. Design of Experimental Set Up

Chapter 3

Design of Experimental Set Up

3.1Introduction

This Chapter describes the design and development of the experimental set up used in the present ice nucleation work. The experimental set up consists of three systems,i)Thermal Gradient Diffusion Chamber (TGDC), ii) Heating unit and iii) Optics unit, with each individual system made up of different components. Section 3.2 describes the specifications of the components involved in the experimental set up development. In Section 3.3 different concepts visualized for TGDC before the final geometry was determined are discussed. Also the suitable construction material for the TGDC and the method of introducing the aerosol is discussed. The Section 3.4 discusses the heating unit, which consists of the PID controller, heating mat and temperature sensors to control and maintain the TGDC system as per requirements. Finally the Section 3.5 discusses the optics system, with description of components involved in the insulation of the microscope, optical fiber cable (OFC) and microscope objective lens cover tube.

3.2Establishing specifications

The target specifications in the design and development of the experimental set up are listed as follows;

TGDC system:

• The developed system should have the facilityto introduce different aerosol particles and expose them to controlled conditionsof temperature and humidity.
• The instrument should be capable of operating inside the Environmental cold Room (ER) which uses forced convection of heat transfer to raise or lower the temperature.
• The geometric size of the developed instrument should to be compatible with the internal dimensions of the ER.
• Steady temperatures for the TGDC for the required experimental time are desirable thus maintaining the temperature stability is an important criterion.
• A cylindrical shape of the TGDC is preferred over the rectangular shape chambers, because,

a)The cylindrical chambersare easy to construct with few joints and parts,

b)Permit the cylindrical chambers to withstand higher pressure differences between the interior and the surrounding atmosphere,

c)Cylindrical chambers are easy to manufacture,

Heating system:

The components involved in this system should respond fast and accurately to any temperature change. A control loop feedback system is needed to be implemented for greater accuracy.

Optics system:

The phase transition or the nucleation phenomenon of aerosol particles once exposed to the determined thermodynamic conditions (temperature, humidity and pressure) needs to be observed in real time. Also it is required to observe the micron sized aerosol particle with high working distance microscope. Observationsare performed using an optical microscope with the following operating conditions taken into consideration.

a) The field of view and working distance decreases as optical resolution increases,

b)Requirement of a cold light source to minimise energy used for illuminating the ice crystal sample,

c)A working temperature range from -5.0 to +30.0 ˚C.

Further to these requirements images observed needed to be saved to the hard drive of the computer.

3.3Thermal Gradient Diffusion Chamber development

In the TGDC development phase the concepts were generated and evaluated according to the experimental set up specifications. Also different components involved in the development are described.

3.3.1Concept generation

The concept generation process begins with a set of needs and target specifications and results in a set of concepts in the form of a schematic diagram or rough sketch. Each concept is then analyzed for its feasibility and is improved in the subsequent concept development phase.

3.3.1.1Evolution of TGDC design concepts

TGDC design concept stage 1:

After a discussion and working with the initial rough sketches, a first concept design layout was produced. Figure 3.1 shows a schematic diagram of the TGDC and figure 3.2 shows the magnified view of the optical assembly. The microscope is focused over the sample area to observe the nucleation of aerosolsunder different supersaturations with respect to ice (SSi)conditions. These conditions are obtained by varying the temperature difference T1 and T2, where T1 is the temperaturemaintained in equilibrium with ER ambient temperature and T1 T2 . The other temperature T2is varied by circulating the coolant, which cools the sample base.

Figure 3.1: Schematic layout of TGDC (not to scale). Fig. a) shows the front view of the TGDC with different parts involved in making the assembly. Fig. b) shows the cross sectional view at A-A plane of the front view.

Figure 3.2:Magnified view of the microscope assembly. The light source illuminates the aerosol sample situated inside the TGDC and the motor pumps the warm air near the objective lens of the microscope to prevent condensation on the inside surface of the glass window.

The layout of the TGDC system has many parts, with both advantages and disadvantages. If there are many parts, multiple parts allow modification of the overall geometry and give flexibility in the selection of different materials. However, multiple parts induce inhomogeneous thermal distribution, take time to assemble, and introduce difficulties in controlling the overall performance of the system. Nucleation phenomena are very temperature dependent and uniform thermal distribution is desirable. Thus it is proposed to reduce the number of parts involved in the present TGDC concept.

In the present layout (Fig 3.1) water vapour fields at different subzero temperatures are produced by the frozen filter paper. Initially the filter papers are saturated with water, and exposed to subzero temperatures to obtain the frozen filter paper.This method of producing water vapour fields suffers from uncertainty in the homogeneous distribution of ice inside the filter paperand how efficiently they stick to the metallic surfaces of the TGDC. If the ice density is not homogeneously distributed across the filter paper, then it will produce non uniform SSi values inside the TGDC. This will introduce errors duringinvestigation of the nucleation results. Therefore a new way of producing the water vapour fields needed to be investigated.

The introduction of the sample is performed either by taking apart the optical assembly or lowering down the heat removal system. This procedure of introducing the aerosol samples is cumbersome and time consuming. Also, the introduction of the sample in the experiments is performed manually at subzero temperatures inside the ER and is therefore unfeasible. This feature is further improved in the next concept development phase.

The construction of the optical assembly is complex in a way to accommodate the motor, to circulate the air near the microscope, and light source. Observation of the nucleation phenomenon at different SSi values is performed by raising or lowering simultaneously the sample base and the microscope. The interface between the microscope and TGDC in the present concept is impractical from an operating point of view. New interface design layouts are needed and are described in the next concept development phases.

These concept design refinements are summarized as follows for further modification of the TGDC concept,

1)Modify the developed concept geometry to have fewer parts.

2)Investigate new ways of producing the water vapour field inside the TGDC.

3)Design a simpler method of introducing the aerosol sample.

4)The interface and in house optical assembly needed further work to make it simpler.

TGDC design concept stage 2:

A new conceptwasdeveloped after considering these identified useful design refinements and is shown in the Figure 3.3.

ER

M

G

Top plate

WV

IL

Bottom Plate

GR

S

Figure3.3: The schematic experimental apparatus of TGDC (not toscale). There are fewer components compared to the first concept geometry. To maintain the temperature difference between the two plates, the top plate is heated using a coil and the bottom plate is in equilibrium with ER ambient temperature.

In this concept design, two parallel plates (top and bottom plates) are used to maintain the two different temperatures T1 and T2. The top plate (T1) has a provision to fit a glass window for the microscope to view the nucleation events and also adjacent to this is aheating coil. The bottom plateholds the assembly for introducing the aerosol sample inside the TGDC. Both plates are ice coated from the inside and in the system their temperatures can be controlled independently. Also they are insulated from each other by a silicon rubber gasket. The heating coil associated with the top plate is used to raise the temperature to the desired value (T1). This temperature difference, (T1T2), creates a net diffusion gradient across the two plates and the water vapour diffuses towards the bottom plate. Aerosol samples are inserted on top of the glass rod to expose the aerosol sample to different SSi conditions. The glass rod can slide up and down.

The thermal and vapour diffusion distribution inside the chamber depends upon the geometry and material of the system used. The ER uses a forced convection type of heat transfer mechanism to cool the TGDC. Due to low thermal conductivity of the air, the systems designed to operate inside the ER should have large mass to avoid temperature fluctuations. So that bulky systems can maintain the constant temperature for longer time or over the duration of the experimental time. From the heat equation, Eqn. 3.1, the heat absorbed or lost (q) is related to the mass of the system (m), specific heat capacity(Cp) of the system and the temperature difference (dT) between the system and it’s surroundings, defined as:

q = m x Cp xdT Eqn. 3.1

From this equation, the choice of material for the chamber system would have a large m and Cp value. Large m and Cp value materials produce thermal stability, thus producing constant SSi spectrum values inside the chamber while performing the experiments. On the other hand large m value material will lower the cooling rate of the chamber system and experiment might take longer time to run. The optimized value of m is needed to be determined using simple heat transfer calculations and modeling CFD simulations.

The diffusion distribution also depends upon the geometry of the chamber system. If the geometry is complex and uniform thermal distribution is required, then a, low thermal conductivity (K) material is preferable. The final geometry design of the top plate depends upon the interface between the microscope and the chamber system, and is improved in the next concept development phase.

In Section 3.2 (microscope design considerations) it was understood the maximum working distance of the available microscope in the market for the budget was 33 mm. This was an important design constraint considered in further geometry design analysis.

To summarize, the following are the further refinementsneeded to improve the present concept;

1)Selection of materials which have high m, Cp and low K.

2)Revisit the interface design geometry between the chamber system and the optical assembly.

3)Consider the working distance of the microscope and insulation to finalize the overall geometry of the chamber system.

4)To provide the provision for the experimental set up to work at pressures lower than the atmospheric pressure.

TGDC design concept stage 3:

The new concept is developed considering the key design factors from the previous concept and is shown in Figure 3.4.

Figure 3.4:Original schematichand sketch of third TGDC developed in the concept phase development process. Rack and pinion arrangement is used to raise the sample inside the chamber. All dimensions in mm.

The technique of producing an SSi spectrum inside the chamber system is the same as discussed in the second concept. Here the interface geometry design is made feasible and simpler. The interface top plate has a cylindrical groove just a few mm more than the diameter of the objective lens of the microscope. This groove serves as an optical port to the chamber system. A glass window fits inside the optical port to prevent any leaks of water vapour outside the system. It was also decided to shield the objective lens with the plastic cylindrical tube from the subzero temperatures. Bush rings are placed inside the two plates as shown in the figure to ease preparation of the ice layers. There is also concern that the contact between the bush rings and system may induce non-uniform thermal distributions and affect the SSi spectrum.

The similar method of introducing the sample is used as explained in the previous concept. More ideas are generated on how to raise or lower the aerosol sample and one idea of doing this is to use the rack and pinion arrangement as shown in

Figure3.4.

Four diametrically opposite threaded port locations are shown on the bottom plate to carry out the experiments at sub-atmospheric pressure conditions. These ports can be connected to an extraction pump to extract air and water vapour from inside the chamber system. Using this procedure, pressure can be dropped inside the chamber system and experiments can be performed at the desiredatmospheric conditions.

Although the present concept developed has less parts and a more simple geometry,it still has some drawbacks and there is potential to make it simpler. The drawbacks are the influence of the bush rings on the overall thermal distribution and the feasibility of the rack and pinion arrangement in the ER. These areas are improved in the next concept development process.

3.3.1.2TGDC final design concept

After many iterations of the design process a final concept was produced which met the needs of the present research and satisfied the target specifications. Figure 3.5 illustrates the geometry of the final concept along with the simplified interface optical assembly design.

Figure3.5: The final experimental TGDC geometry with provision for temperature sensor (RTD) locations and a heating mat. The microscope is raised and lowered inside the objective lens cover tube. Scale 1:2 and dimensions in mm.

The final TGDC set up (see Fig 3.5) consists of two horizontal parallel plates, with the assembly of the top and bottom plates separated by a thin, 1mm thick rubber gasket. The top plate has a port at the centre where a glass window has been installed, which allows illumination and observation of the sample by the microscope and allows the objective lens to protrude inside the port. The bottom plate has four circular diametrically opposite threaded ports at its side edges which can be used to pump gas in or out of the chamber.

Temperature measurements are made at four locations, shown as RTD locations in Fig. 3.5, on each of the chamber plates with the RTD to 0.1 degree accuracy. A circular shaped heating pad has been affixed to the outer surface of the upper plate in order to control the temperature independently during the operation of the chamber, whereas the temperature of lower plate is at the ambient ER

temperature (T2).

A simpler way of making the ice layers inside the chamber compared to the previous concept was determined. The two plates from the assembly are separated, inverted and de-ionized water of constant volume is poured into the plates. The plates are then cooledto freeze the water.Thus using constant volume of water the thickness of the ice layers is maintained constant in all the experiments and thus the height between the two ice layers is constant, as the geometry of the TGDC is fixed.

The microscope assembly is maintained at a non freezing temperature by heating and suitable insulation, and is aligned with the chamber for observation. Both the microscope and chamber assembly are kept inside the ER.

3.3.2Construction materialfor TGDC

Construction elements of normal chambers include windows, usually of glass or Perspex.For the body and sealing materials stainless steel, brass, aluminium or copperare typically used.

For the selection of chamber body material different criteriawere considered as in Eqn. 3.1. In the present situation, the chamber is exposed to humid conditions and therefore the material should be corrosion resistant. Stainless steel grade 304L has very good corrosive resistance property, whereas the aluminium and copper materials form a thin layer of oxidation after some time. This increases the risk of contamination and should be cleaned thoroughly every time the experiment is conducted. This also increases experimental time and is a cumbersome process.

Low thermal conductivity value (K) materials offer higher thermal resistance and produce uniform thermal distributions. In the case of copper, the heat applied to a region at the surface of the top plate, would be transferred so rapidly that it wouldnot have time to diffuse uniformly across the top plate.

In the current chamber geometry near the end of the observation port (end of objective lens of the microscope) of the top plate, there is no direct heating, and therefore to produce a uniform temperature distribution it is necessary to use a material of low thermal conductivity which diffuses the heat uniformly. Table 3.1 compares the thermodynamic values of different materials. The obvious choice for the chamber material is the stainless steel 304 L grade material.

Table 3.1: Showing thermodynamic properties of different materials. Steel has high Cp and density with low thermal conductivity (K) making the material suitable for the chamber development.

Material / Stainless Steel
304L grade / Aluminum / Copper
K (W/m K) / 16 / 235 / 400
Density (Kg/m3) / 7900 / 2702 / 8933
Specific heat capacity, Cp(J/KgK) / 477 / 903 / 385

A combination of high density and heat capacity of the stainless steel 304 L grade makes the material thermally stable. The importance of performing the experiments at thermal equilibrium conditions is explained in the concept development phase.