Figure 1: Hot wire generator
2.2 Chemical characterization of wire material & its evaporation rate
As evaporation rate of heated wire forms the basis of vapor source term which subsequently nucleates, the chemical composition of wire is required to be known precisely. The saturation vapor pressure of constituents of wire is one important input for calculating its evaporation rate. Elemental composition of the tested wires has been obtained using XRF analysis. Following table shows the elemental composition of two wire materials used for this study. Conventional electrical heater wire was used as 1st wire (wire A) while commercial grade nichrome wire was the 2nd wire (wire B) for all experiments.
Evaporation rate (r) for these wires can be calculated using Hertz–Knutsen equation under some assumptions
Where λ= accommodation coefficient, M = molar mass, T = temperature, ps (T) = saturation vapour pressure, p0 = vapour pressure, Na= Avogadro number and kb = Boltzmann constant.
In above equation, accommodation coefficient accounts for vapour scattering back to the wire surface (for that correction, λ should be lesser than 1), however it can safely be assumed to be equal to 1 for the generation conditions. Since a carrier flow was used in the system, p0 can also be assumed to be zero.
Saturation vapor pressure for these elements can be taken from standard vapor pressure look up tables. In absence of the measurement of the surface temperature of the hot wire, melting point of the constituent was taken as T in the calculations. Table 2 shows the calculated ps values and the corresponding evaporation rate for elements which were obtained by XRF analysis of these wires.
From the table 2, it is evident that chromium contributes maximum towards vapor generation for present wires/study cases (percentage of Mn was very low for both wires and vapor pressure of Fe is significantly lower compared to Cr). The value shown in the table are upper bound of the evaporation rate as they are estimated based on individual metal melting point instead of alloy melting point (1673 K). Considering Cr as the vaporized constituent (100%), calculated evaporation rate (mole m-2s-1) can be converted to mole m-2s-1 which subsequently can be used to calculate source emission rate (SE) using the following equation:
Where, S is total surface area of the evaporating material and V is generator volume. In present study, evaporation rate of wire material was estimated ∼5.78 x 1024 mole m-2s-1 while source emission rate (for generator volume of 450 cm3) was ≈ 2.0 x 1025 mole m-3s-1.
3. Results and discussions
3.1 Optimization of Hot Wire Generator’s parameters
A series of experiments have been carried out for obtaining optimum operating conditions for generation of stable high aerosol number concentrations. Results indicated that carrier gas flow rate and applied voltage are two main factors affecting the generator output.
Carrier gas flow rate plays an important role for generation and transport of the aerosols. During the generation process hot vapors carried by a cooler carrier gas become supersaturated, and nucleate and to form critical size nuclei. In presence of flow, residence time of vapor in hot regions changes, resulting in stable generator output.
Table 1: Elemental Composition of wire material used in experiments
| Components |
Weight % of components (Wire A) |
Weight % of components (Wire B) |
| Cr |
18.2±0.1% |
17.6±0.2 % |
| Fe |
78.2±0.3% |
0.4±0.1 % |
| Ni |
ND |
79.8±0.3% |
| Mn |
3.2±0.1 % |
2.1±0.1 % |
Table 2: Thermo-physical properties of wire elements
| Element |
Melting Temperature, Tm (K) |
Ps(Tm), Pa |
Evaporation rate, r (mole m-2s-1) |
| Cr |
2180 |
7.39 x102 |
9.6 |
| Fe |
1811 |
3.39 |
4.66 x10-2 |
| Ni |
1728 |
1.04 x10-1 |
1.42 x10-3 |
| Mn |
1519 |
1.42 x102 |
2.15 |
