ILASS Americas 28th Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May 2016

Rapid Evaporation of Water Sprayed on Metallic Media Beds

D. J. Bouchard, and S. Chandra*

Department of Mechanical Engineering University of Toronto

Toronto, ON M5S 3G8, Canada

Abstract

Rapid evaporation of water for steam generation can be achieved by spraying liquid droplets on a heated metallic surface. This paper describes an experimental study to measure the rate at which water sprayed on the surface of a metallic media bed evaporates. The experimental test apparatus consists of a balance, one pan of which is filled with the metallic media while the other pan acts as a counterweight. The metal is heated electrically to the desired initial temperature, after which the heater is switched off and a water spray is directed on the hot surface. A digital force gauge is used to measure the displacement of the balance arm as a function of time, from which the mass of water in the pan is determined. This change in mass is used to quantify the rate that the water evaporates, and to study the influence that different parameters have on the evaporation rate of water. The parameters studied in this paper include the thermal mass of the media bed and the total surface area. Increasing the surface area does not necessarily increase the rate of water evaporation. The impact and evaporation of water droplets on the hot surface is captured using high-speed photography.

*Corresponding author: chandra@mie.utoronto.ca

Introduction

Many industrial applications such as humidifiers re-quire the rapid production of a large volume of steam. One method of doing this is to spray water on a heated mass. The amount of material required is determined by its thermal mass, which depends on the material’s den-sity and specific heat, and the temperature to which it is superheated. Equally important is how the material is distributed. Ideally, it should have a high surface area on which water can evaporate while minimizing the dis-tance through which heat must be conducted within the solid material. A porous media bed satisfies these criteria by having small particles and a high surface area.

Spray cooling can achieve high heat transfer rates due both to convection and the latent energy required for evaporation. Heat transfer during spray cooling can be enhanced by increasing the fluid flow rate, varying the impact angle, or increasing the surface roughness [1]. Applying a microporous coating can also significantly increase the heat transfer rate of a flat surface [2]. Ma-chined surface structures such as pin arrays [3] or micro pyramids [4] have also been tested. The enhancement observed for structured surfaces is thought to be partly due to the increase in surface area and the structures be-ing larger than the liquid film thickness. This increases the three–phase contact line length, which increases the rate of evaporation.

Coursey et al. [5] tested 500µm thick fins with heights varying from 0.25mm to 5mm. The finned sur-faces entered the two-phase heat transfer regime at lower temperatures than a flat surface. The authors speculate this may be caused by the increase in liquid residence time, spray shadowing in the outer channels, and an in-crease in the number of potential nucleation sites. Souza and Barbosa Jr. [6] measured the heat flux for plain and copper foam covered copper surfaces, and obtained en-hancement factors as high as 1.39. Although the im-provement of heat transfer in recent studies is quantified, the amount of vapour generated is not a focus. Addition-ally, research in spray cooling often involves substrates that undergo constant heating instead of a preheated mass.

Our goal in this study is to generate steam from 4-8 l of water in as short of a time as possible, preferably under 10 s. To quantify the rate of evaporation from po-rous media beds, a test apparatus was constructed to measure the amount of water remaining in a scaled down media bed after it was sprayed. This paper describes the apparatus as well as presents some of the initial results produced with it. The rate of water percolation into the media bed is found to be an important factor when trying to maximize the amount of water evaporated in a fixed length of time.

Materials

The spray stand was constructed using 40mm alu-minum T-slotted framing and brackets (McMaster-Carr). The spray stand held the spray nozzle (1/8HGS4.3W: BEX) and a two-way solenoid valve (8262H232: ASCO) connected to a rotary vane pump (500-1000 Series: Fluid-o-Tech) and a water reservoir. Figure 1 shows the arrangement of the components in the pumping circuit. A toggle switch was installed in series between the sole-noid valve and the DC power supply (PR3UL: Tripp Lite) for manually opening and closing the solenoid valve.

Figure 1. Schematic of the components within the wa-

ter pumping circuit

The evaporation balance consists of a weighing pan, a counter-weight pan, and a digital force gauge (DFG55-2: Omega), Figure 2. Forty millimeter aluminum T-slot-ted framing and brackets were used to support the com-ponents. The weighing pan is 225 mm in diameter and is hung from a 125Øx6.4 mm steel disc using 1.59 mm 316 stainless steel wire rope. Each wire rope is spliced with a light-duty turnbuckle, which are used to level the pan. The connection points to the weighing pan and the steel disc are spaced 120° apart from each other.

Figure 2. Evaporation balance schematic.

The balance arm connecting the two pans is a square 25x25x965 mm long 6061-T6 aluminum bar. A 3.18 mm bullnose groove was machined across the bottom of the bar at a point 450 mm from the weighing pan end of the bar. This groove is placed on a 90° track roller rail (5/8” high by ¼” wide: McMaster-Carr), which acts as the pivot point for the balance arm.

The counter-weight pan is 225 mm in diameter and is attached to the opposite end of the balance arm with a custom aluminum bracket. Two 8x12.75 mm elliptical slots were machined through the balance arm at an equal spacing of 100 mm from the bullnose groove. Threaded rod (¼-20) goes through these slots to act as safety guides as well as being used for leveling during the ex-perimental setup.

The digital force gauge is mounted to a 150x150x6.35 mm thick steel plate, which is bolted to two 40 mm T-slotted aluminum uprights. A holding pin is attached to the side of one upright to secure the balance arm during sample heating. The load cell shaft of the dig-ital force gauge contacts the balance arm at a distance of 300 mm from the pivot point. The distances of the center of the weighing pan and the counter-weight pan from the pivot point are 438 mm and 648 mm, respectively.

High-speed photography was performed with a Pho-ton FASTCAM SA5 with an AF Micro Nikkor 60mm, 1:2.8D lens. The light source is a 250 W halogen bulb mounted in a LowelPro spot light. A sheet of Mylar was used as a light diffuser.

A 1000 W residential hotplate was placed on a 200x200 mm laboratory jack for height adjustment. A type-K thermocouple (Omega) was welded to an 8 mm steel sphere and placed on top of the media during heat-ing. It was connected to a thermocouple reader (HH12: Omega) to measure the surface temperature of the media bed.

Aluminum rods were used at the basis of compari-son for this study. For a comparison of the amount of steam generation, carbon steel spheres 16mm and 8mm in diameter were counted to equal either the thermal mass (16mm spheres-TM and 8mm spheres-TM) or the surface area (16mm spheres-SA and 8mm spheres-SA) of the aluminum rods. Steel shot was weighed to equal the thermal mass of the aluminum rods only. The rele-vant properties of these six different media beds are sum-marized in Table 1.

To contain sprayed water within the weighing pan during each test, a dome-shaped aluminum foil cover was made and placed on the weighing pan prior to heat-ing. The dome’s height was adjusted so the opening at the top of the dome was just below the spray nozzle tip. Test Procedure

The settings of the data logging software (MESUR Lite: Mark-10) are: 10 samples per second, 1000 meas-urements, trigger load of 15gF. The digital force gauge was set to a data collection rate of 10 samples per second, and the built-in moving average filter was set to 256. Each spray trial was performed using the following steps:

1) Fill with reservoir with tap water and prime the tubing with water. While the pump is operating, adjust the flow rate to 1.89 lpm.

2) Fill the weighing pan with the desired media and place the balance arm on the pivot point. Adjust the nuts on the threaded rods to level the balance arm, and then level the weighing pan using the turn buckles.

3) Adjust the height of the spray nozzle tip to be 108mm above the top surface of the media bed. At this height the nozzle will have a 225mm di-ameter spray coverage at the surface of the me-dia bed.

4) Add the aluminum foil cover to the weighing pan, and centre the spray nozzle over the media bed.

5) Add mass to the counter-weight pan until the weight of the weighing pan is balanced.

6) Attach the digital load gauge to the steel mount-ing plate and connect it to a data collection lap-top.

7) Trim the mass in the counter-weight pan until the load gauge reads 0-5 gF.

Media Material Total Mass

[kg] Surface Area

[m2] Number of layers

Thermal Mass [kJ/K]

Al Rods 16mm diameter, 6061-T6 aluminum rods

3.7 0.361 3 3.28

16mm

sphere-TM

16mm diameter, 1010/1020 carbon steel spheres

6.6 0.323 2.0 3.21

16mm

spheres-SA

16mm diameter, 1010/1020 carbon steel spheres

7.4 0.363 2.5 3.6

8mm

spheres-TM

8mm diameter 1010/1020 carbon steel spheres

6.7 0.642 4.2 3.28

8mm

spheres-SA

8mm diameter 1010/1020 carbon steel spheres

3.7 0.362 2.3 1.8

Steel shot 0.5-0.7mm diameter 1020 carbon steel

6.7 8.56 32mm 3.27

Table 1. Properties of metallic media beds

8) Engage the holding pin and place the thermo-couple on the media bed. Raise heater and com-mence the heating of the sample.

9) When the surface of the media bed reaches 200°C, remove the thermocouple, lower the heater, and disengage the holding pin. Ensure the load gauge reads less than 5gF. If not, adjust the mass in the counter-weight pan.

10) Start the pump and the data collection program. 11) Manually trigger the spray for 4 s. This amount

of time will spray approximately 130ml of wa-ter into the weighing pan.

Data Processing

The raw data obtained from a typical spray trial is shown in Figure 3. The raw data is processed in three steps. First, to make the data more intuitive to read, the mass of the water remaining in the weighing pan must be converted from a force to a volume of water. Second, the digital force gauge also registers the impact force of the spray, which should be removed because the impact force makes it seem like more water is in the pan than there actually is. Third, as the water hits the superheated porous media bed, it boils and bursting vapor bubbles create vibrations that are measured by the digital force gauge. The vibrations should be smoothed out to make trends more visible.

Figure 3. Representative unprocessed data obtained

from a typical spray trial.

To convert the force displayed by the digital force gauge to a volume of water, a beaker was placed in the centre of the weighing pan and filled with water 1 ml at time. The average force displayed on the digital force gauge was recorded for each milliliter of water added up to 10 ml. A linear regression of the points create a line with a slope of 1.44 gF/ml of water. Calculating what the force should be using the distances of the moment arms from the pivot point and the density of water (1g/ml) re-sults conversion of 1.46gF/ml This is close to the result

of the linear regression and indicates that the pivot point provides very little resistance in the system. The data presented in this paper was converted from gF to ml of water using the conversion of 1.44 gF/ml.

The total force exerted by the momentum of the im-pinging water droplets was measured by spraying the un-heated weighing pan and recording the force measure-ment from the digital force gauge. The volume of water sprayed per second was calculated based on the meas-ured flow rate and spray duration, and then multiplied by the conversion factor of 1.44gF/ml. The difference be-tween the total force of the spray and the weight of the volume of water in the weighing pan is the impact force of the spray, Figure 4. It is suspected that the impact force slowly decreases from its maximum at the start of the spray because the accumulating water in the pan dis-sipates some of the momentum of the spray. An average impact force of 33.43 gF (N=7, SD=3.28) is assumed for all of the trials and is removed from the spray cycle of the experiment before converting the measured force to a mass of water.

Figure 4. Determination of impact force generated by the spray nozzle. The weight of the water in the weigh-

ing pan is subtracted from the total force read by the digital force gauge to arrive at the impact force.

To smooth the data, a 5-point central moving aver-age was used. Figure 5 shows the same data presented in Figure 3 with the three aforementioned processing steps. The figures in the remainder of the paper will be pro-cessed in the same manner.

Figure 5. Raw data in Figure 3 processed to convert grams force into unevaporated water, removal of the spray impact force, and smoothing the data with a 5-

point central moving average. Aluminum Rod Media Bed Results

Aluminum rods were tested first to form a basis of com-parison. The aluminum rods were cut into 29-150 mm long pieces, 12-125 mm pieces, and 12-75 mm pieces and arranged in three layers within the weighing pan.

Figure 6 shows the rod arrangement for the bottom layer. The rods of the subsequent layers were placed in the hollows created by two adjacent rods from the layer below.

Figure 6. Arrangement of aluminum rods for the bot-

tom layer in the weighing pan.

The results from three spray trials using the alumi-num rod media bed is shown in Figure 7. Of the 130 ml that was sprayed, about 30 ml evaporated before the spray cycle was complete. After 10s, approximately 50% of the sprayed water evaporated from the weighing pan. The aluminum rods are packed tightly into the weighing pan, which leaves little room for the water to flow down in between the rods. It is suspected that the top and bot-tom layers do the majority of the evaporating. The mid-dle layer may still contain much of its initial heat because it is protected from the water spray by the top layer and

elevated out of the water that collects at the bottom of the weighing pan by the bottom layer. Changing from rods to spherical media will introduce more water flow paths directly through the media bed.

Figure 7. Results for three replicates of evaporation

from an aluminum rod media bed.

Spherical Media Bed Results

The 16 mm and 8 mm carbon steel spheres were counted and stacked to achieve a close-packed structure. For both of the sphere sizes, a media bed that equaled the aluminum rods either in thermal mass or in surface area was made. If a complete layer could not be made, the remaining spheres in the last layer were scattered ran-domly over the top surface. Steel shot was weighed to a mass that would achieve the same thermal mass as the aluminum rods. A mass of steel shot that would equal the surface area of the aluminum rods is only 290 g. Since this mass is very small compared to the thermal mass of steel required to evaporate 130 ml of water, a test with steel shot having an equivalent surface area to the alumi-num rods was not performed. Each media bed was tested three times. One replicate from each experiment is shown along with one of the aluminum rod replicates in Figure 8.

Despite having an equal thermal mass and a much greater surface area than the aluminum rods, the steel shot evaporated only 30 ml (23%) of the sprayed water after 30s. The reason for this is evident from the high-speed images taken during the experiment. Once the sur-face was cooled by the initial water spray, the water started to pool, Figure 9. The volume of water sprayed on the media bed was faster than both the rate of evapo-ration and the rate of percolation, so the top layers be-came flooded with water.

Figure 10 shows the extent of pooling shortly after the end of the spray. Eventually, this water percolated into the media bed, but significant evaporation of the sprayed water did not occur.

Figure 9. Surface of steel shot media bed at 0.577s af-ter the start of spray. Water begins to pool at centre left

of the image.

The 8mm spheres-TM media bed has a surface area equal to 178% of the aluminum rod’s surface area. Water pooling also occurred on this media bed, but despite this occurring, it performed similarly to the aluminum rods and even evaporated slightly more water at 10s. The 16mm spheres-TM media bed has slightly less surface area than the aluminum rods and it performed the best, evaporating 40 ml and 20 ml more water by the end of the spray and 10s, respectively.

However, when the 16 mm and 8 mm sphere media beds are compared on an equal surface area basis the re-sults are reverse. As seen in Figure 11, the 16mm spheres-SA evaporated less water than both the alumi-num rods and the 8mm spheres-SA media beds at both the end of spray and at 10s. The 8mm spheres-SA evap-orated the most water in less than 10s, which is surpris-ing because it had only 55% of the thermal mass as the aluminum rod media bed.

Figure 10. Steel shot media bed 4.186s after the start of spray (8ms after the end of spray). Steel shot media bed is completely flooded with water. Some evapora-tion still occurring from the deeper levels of the media bed as evidence from the vapour bubbles at the center

of the image.

Although the 8mm spheres-SA only had 55% of the thermal mass as the 8mm spheres-TM, both media beds evaporated a similar amount of water at 10s, Figure 13. At times less than 10s, the 8mm sphere-SA media bed evaporated more water. The 8mm spheres-TM media bed had two more layers of spheres than the 8mm spheres-SA media bed, which caused the water to perco-late into the bed more slowly, Figure 12. The images show the amount of pooled water at the surface of these media beds shortly after the spray stopped.

Figure 8. Comparison of the metallic media beds with equivalent thermal mass.

0

25

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75

100

125

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Un

eva

po

rate

d W

ate

r (m

l)

Time (s)

Steel Shot

Al Rods 2

8mm Spheres-TM

16mm Spheres-TM

Figure 13. Comparison of the unevaporated water for the 8mm spheres-TM and the 8mm spheres-SA media.

Having fewer layers in the media bed also improved the evaporation rate for the 16mm spheres-TM compared to the 16mm spheres-SA, Figure 14. In this case, the 16mm spheres-TM show a 50% improvement despite having less thermal mass and only half of a layer less media. More work is required to understand if other factors are contributing to this performance improvement such as the benefit of fewer larger water flow paths through the media bed than would exist in the 8mm sphere media bed.

Figure 11. Comparison of the metallic media beds with equivalent surface areas.

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50

75

100

125

150

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Un

eva

po

rate

d W

ate

r (m

l)

Time (s)

(a) (b)

8mm Spheres-SA

16mm Spheres-SA

Al Rods 2

Figure 12. The 8mm spheres-SA (a) and 8mm spheres-TM media bed (b) at 4.3s. Less water pooling is evident in the 8mm spheres-SA media bed

Figure 14. Comparison of the unevaporated water in the 16mm spheres-TM and the 16mm spheres-SA me-

dia beds

Conclusion

The above results show that solely increasing the surface area of the thermal mass does not guarantee an increase in the steam generation rate. The size, shape, ar-rangement and spacing of the media must also be con-sidered because the surfaces will quickly cool and then the rate of penetration of liquid into the media bed will start to become a factor.

This work describes an apparatus that can be used to quantify the evaporation rate of water from spray-cooled media and highlights the importance of having a fast per-colation rate. When attempting to evaporate water quickly from a porous media bed, the percolation rate can be a more important than thermal mass. Future work will investigate the effect of flow rate and average drop-let size using different nozzle sizes as well as additional arrangements of thermal mass.

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