Introducció i conceptes

The measurement of the entrained air velocity was performed according to the set up shown in Fig. 4.2, using a High pressure pump to delivered water through the atomizer via a pressure gauge for ensuring the targeted pressure was achieved. The hot wire probe set around a grid lines corresponding to the measurement position, and the air velocities were then displayed. The water tank collects and recycles the water through the pump during the test. A Hot-wire sensor was kept 5mm away along the edge of the spray end to avoid destruction by the High pressure water jet and interference causes by condensing water heat transfer along the hot wire. Axial and radial measurements were performed as shown in the Fig. 4.2.

78 4.2.2 Experimental apparatus

The measurement of the entrained-air around a high pressure water sprays includes apparatus connected together as shown in Fig. 4.2 to carry out the measurement, the following are the main apparatus use in this experiments

Hot Wire Anemometer

The application of a Hot-wire anemometer has been drawn based on the principle of convective heat transfer across a heated sensing element; which is generated upon perturbed by flow unto its surface, leading to changes in the heat transfer coefficient of resistance. It is currently applied in many industrial applications despite the availability of other non- intrusive measurement methods such as multi-component laser Doppler Velocimetry, still two other advantages such as (i) measuring accurately entropy changes and (ii) its capability to measure flow parameters. It operates on the basic principle that electrical output can be established by heat transfer from the cold surrounding air to the heated wire; as such the heat transfer which is a function of fluid velocity can then be accurately measured, which the electrical circuit is used to provide controlled amount of current to the wire to maintain a constant voltage. Although to maintain a constant temperature, the amount of supplied current may be varied to ensure isothermal conditions despite variation in the heat transfer rates. The simplified view of the Hot-wire section is shown in Fig. 4.3.

Figure 4.3 Hot Wire cross section[116]

Sensor dimensions: Length: 1mm Diameter: 5micrometer Current I Velocity U Current I _Sensor Wire support

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A thin wire is placed across a path of moving cold air, with a velocity U, I, is the current, R, resistance. While a current is passed through the wire, an amount of heat equivalent to I2Rw which, when at equilibrium, will compensate for the heat loss to the surroundings. Whenever the velocity changes, the heat transfer changes as well, and ultimately changes the temperature and a new equilibrium established.

Most practical applications require effective material to be used for the Hot-Wire, therefore the following properties were suggested:

i. High coefficient of Temperature resistance

ii. Suitable electrical resistance which can conveniently heat wire at practical currents and voltages

iii. Availability of wires at very low diameters

iv. Adequate strength to overcome aerodynamic stress even at high velocities

The governing equation for the energy transfer between the Hot wire and the surrounding air is:

W H dt

dE _{ }

(4.1) Where E=Thermal energy stored within the wire CwT, with Cw=heat capacity of thewire

W= Power generated by the Joule heating I2Rw

H=Heat transferred to the surrounding by conduction, convection and radiation.Considering the overall Energy balance equation generated by the heat transfer,





H (Convection to fluids + conduction to supports+ radiation to surroundings (4.2) The convection equation heat transfer is governed by the equation

Q_convNuA(T_wallT_air) (4.3) Where Nu, is the Nusselt Number given by

f k hd

Nu (4.4)

Hence, Eq. 4.1 can be written in terms of static heat transfer as follows:

WHI2R_w hA(T_wallT_air) (4.5) Leading to transformation in form of dimensionless Nu, as

80 2 _w f (T_wall T_air) d A Nuk R I   (4.6) For Forced convection heat transfer, where ReGr1/3 having a value between 0.02 in air and Re140 ) )( ( 2 2 n air wall w E T T A B U R I      (4.7)

According to King’s law[116].

4.2.3 Experimental procedure

The connection between the digital display and the hot wire were mounted on the experimental rig as shown in Fig. 4.6 and placed, according to the grid established in Section 4.3. The pump was then primed and started, with subsequent pressure increases until the desired pressure was established using the pressure gauge on the nozzle top. The flow was then maintained for 2 minutes until stability of the water and air flow was achieved. The reading of the air velocities was then taken for 10 seconds to ensure accuracy and then subsequently repeated for axial positions at 25, 50 and 75mm and radial positions from 0, 5, 10, 15 and 20mm.Then it was traversed for -5, 10, -15 and -20mm The results of the air velocity around the spray are shown in Section 5.2.2.2.

Figure 4.4 Entrained-air velocities measurement grid

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1. The probe was connected to the probe input socket 2. The power on the meter was turn On

3. The unit for the velocity and temperature were chosen

4. The sensor slide cover was slid up to isolate the air velocity from the hot wire until the zero reading was observed at the display

5. The velocity meter was then zeroed at the isolation instance and then 6. The cover was slid down to allow air contact for the velocity measurement

4.2.4 Accuracy and Errors in Hot-Wire Anemometry

4.2.4.1 Precautions and accuracies adopted during the experiments  Repetitive sensor drying to minimize water droplets fouling  Liquid droplets not allowed to cascade in air

 Spray was allowed to stabilise before readings were taken

 Water vapour condensation was minimized on the hot wire during the experiments 4.2.4.2 Sources of errors

 Probe contaminations in air: The presence of dust, vapours, dirt’s or chemicals affect the flow sensitivity of the hot sensor or experience a reduction in frequency of response. It is usually signalled as a drift due to particle contamination from the calibration curve shown in Fig. 4.3. This includes exposure to winter conditions or unfiltered air at 40m/s. Other effects includes low velocity due to the slight effect of dirt on the heat transfer

 Bubbles in liquids: In liquid components, dissolve gases generate bubbles on the sensors leading to reduced heat transfer rate and downward calibration drift.

 Readings taken within ±1mm along X and ±1mm along Y axis.

4.2.5 Hot Wire Calibration

The calibration principle of the Hot Wire Anemometer is derived from the King’s law with its response derived as[116]:

E2 ABUn (4.8) Where E is considered as the voltage across the hot wire,

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U, is the velocity normal to the wire, and A, B and n are constants. Using linear regression and plotting the velocity versus the voltage during the industrial calibration. The measurement of the high pressure water spray characterization in terms of drop size and velocities were conducted using the PDA procedure detailed in Section 4.3.