An experimental method to quantify local air-side heat transfer coefficient through mass transfer measurements utilizing color change coatings

International Journal of Heat and Mass Transfer 144 (2019) 118624

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An experimental method to quantify local air-side heat transfer coefficient through mass transfer measurements utilizing color change coatings Min Che a, Stefan Elbel a,b,⇑ a Air Conditioning and Refrigeration Center, Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, USA b Creative Thermal Solutions, Inc., 2209 North Willow Road, Urbana, IL 61802, USA

a r t i c l e

i n f o

Article history: Received 6 February 2019 Received in revised form 22 August 2019 Accepted 22 August 2019 Available online 30 August 2019 Keywords: Local air-side heat transfer Convection mass transfer Heat and mass transfer analogy Coating Tracer gas Color change

a b s t r a c t This paper presents a visualization method to quantify local air-side heat transfer coefficient (HTC). It is challenging to measure local air-side HTC with good accuracy, especially for complicated geometries and real heat exchangers. The present method relies on measuring convective mass transfer and applying the analogy between heat and mass transfer. It is based on the chemical interaction between a pair of coating material and tracer gas. The coating material absorbs tracer gas and changes its color. Therefore, a visualization procedure is developed to correlate color change on the surface to the local mass transfer coefficient. In this research, the coating formulation, coating methods, and surface topography are evaluated to make sure the analogy between heat and mass transfer is valid. The experimental results of the flat plate in laminar flow show that the standard uncertainty of the local heat transfer coefficient is 15%. Furthermore, the results also show good agreement compared with the analytical Blasius solution except in the vicinity of the leading and trailing edges. Because of these promising results, it seems feasible to use this method to acquire local air-side HTC through a visualization approach for more complicated geometries. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Heat transfer between liquid-to-gas is widely used in aerospace, transportation, electronics cooling, industrial processes, refrigeration, and air-conditioning areas. Therefore, liquid-to-gas heat exchangers are intensively investigated for energy saving considerations. Jacobi and Shah [1] reviewed the air-side enhancement induced by vortex generators. It is shown that trade-offs between increasing HTC, air-side pressure drop penalty, and high manufacturing cost limit air-side performance improvements. Joardar and Jacobi [2] have claimed that air-side convective resistance to be the most significant contributor, which accounts for 75% or more of the total heat transfer resistance in refrigerant-to-air heat exchangers. Due to the complexity of air-side geometries, empirical correlations from experimental studies are usually used to predict heat transfer performance. Therefore, experimental techniques

⇑ Corresponding author at: 1206 West Green Street, Urbana, IL 61801, USA. E-mail address: [email protected] (S. Elbel). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118624 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.

have been intensively studied in the past decades to quantify airside HTC in order to seek opportunities to improve the efficiency of the fluid-to-gas heat exchangers. The Wilson Plot Method has been proposed by Wilson [3] a century ago and is still widely used today. However, it is only possible to obtain averaged HTC with this method. There are three most common technical approaches to measure local HTC experimentally: (i) via temperature measurements; (ii) through flow velocity measurements; (iii) by mass transfer measurements. Furthermore, there are several specific techniques available for each approach. However, it is still challenging to quantify local air-side HTC on complicated geometries and entire heat exchanger surfaces. The purpose of this study is to propose a practical experimental method which has the potential to be applied on complicated surfaces to obtain local air-side HTC. This method employs the analogy between heat and mass transfer. A visualization procedure is developed to quantify local mass transfer by color change observation. Since the method employs a coating material in combination with a tracer gas to achieve a local color change, this method will be abbreviated as CTC in this paper.

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Nomenclature k Symbols A C D h hm H m m00 M Nu Pr q00 Re S Sc Sh Le t T U u v x X y Y z

area of the surface [m2] concentration [kg/m3] diffusion coefficient [m2/s] convection heat transfer coefficient [W/m2 K] convection mass transfer coefficient [m/s] hue: color property [–] mass [kg] mass flux [kg/m2 s] mass of ammonia absorption [g] Nusselt number [–] Prandtl number [–] heat flux [W/m2] Reynolds number [–] color change factor [–] Schmidt number [–] Sherwood number [–] Lewis number (=Sc/Pr) [–] time [s] temperature [°C] bulk flow velocity [m/s] streamwise flow velocity [m/s] flow velocity normal to the plate [m/s] streamwise dimension [m] one independent variable [–] coordinate direction normal to the plate [m] uncertainty [–] spanwise dimension [m]

m
D

q d

fluid thermal conductivity [W/m K] kinematic viscosity [m2/s] ratio of color change [–] difference [–] density [kg/m3] thickness [mm]

Subscripts b blue fc final color i ith variable ic initial color L characteristic length loc local max maximum now at current moment tot total w at the wall 1 free stream flow Abbreviations AAM ammonia absorption method AMS 67 the coating material CTC the proposed method HTC heat transfer coefficient TLC thermochromic liquid crystal HSI hue, saturation, and intensity HSV hue, saturation and value RGB red, green and blue RH relative humidity

Greek symbols a thermal diffusivity [m2/s]

2. Review of experimental methods for measuring air-side heat transfer The main experimental methods to quantify HTC are classified and summarized in Fig. 1. There are three technical approaches to obtain local HTC. For each approach, the most commonly applied technologies are studied and compared. As the CTC method employs a similar color analysis as the Thermochromic Liquid Crystal (TLC) method, more details about the TLC method are reviewed. Furthermore, the CTC method employs the analogy between heat and mass transfer. Therefore, the mass transfer methods, such as the naphthalene sublimation method, the ammonia absorption method (AAM), and the swollen polymer method, are explained in this section. 2.1. TLC method According to Newton’s Law of Cooling, if the temperature difference between the wall and the fluid is measured locally and the heat flux is controlled or measured, the local heat transfer coefficient can be quantified. A Thermochromic Liquid Crystal (TLC) is a material which undergoes molecular structure change as its temperature varies. The observed wavelength of reflected light in the visible range is a function of the molecular structure. Therefore, color observation and temperature can be correlated. By adding a thin film coating of TLC on the solid surface of interest, temperature mapping on the surface is achieved. An optical method for color observation and image processing made this method advantageous for visualizing the heat transfer. Hue is directly related to color wavelength. It is used to describe

color instead of the RGB color space. The reason for using hue to represent color, and the mechanism is explained in detail by Cantrell et al. [4], the process and equations of converting RGB information to HSI are also provided. Besides, the hue values can be easily obtained from RGB images by employing MATLAB. In-situ calibration is required for each specific experiment and application to obtain a correlation between temperature and hue value. Temperature measurement range limits the application of the TLC method. According to Cooper et al. [5], the temperature range of initial narrow-band crystal is only 1 to 2 °C. Therefore, they used eight different crystal materials on the cylinder to measure a temperature range from 32 to 49 °C in 1975. As the crystals have been improved over time, in 2009, Kakade et al. [6] applied a wide-band crystal with a temperature range of 5 to 20 °C for their measurements. However, the bandwidth is still limited for heat exchanger applications. Ireland and Jones [7] mentioned that different types of crystal materials can be pre-mixed to increase the range of temperature measurements. Toriyama et al. [8] recently increased the TLC temperature range from 32 to 42 °C to 27 to 60 °C by observing the spectrum intensity of scattered light instead of the visible light. Although they improved the range and resolution of the temperature measurement, special optical devices, and more complicated image processing are required. Bandwidth is still a limitation of the TLC method. Kakade et al. [6] investigated many factors such as coating thickness, the design of calibration device, illumination configurations, image acquisition, and processing. They found that these factors have a significant influence on the accuracy of temperature measurements. As tested by Ireland and Jones [7], for a 20 mm coating, temperature drop caused by conduction is 2 °C when the heat

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Fig. 1. Selection of experimental methods to quantify air-side HTC. 4

2

flux is 2  10 W/m . Therefore, a suitable coating thickness needs to be controlled, considering thermal resistance, color observation, and maximum heat flux. However, none of the studies have mentioned coating thickness control and evaluation. Furthermore, TLC materials will be destroyed when exposed to ultraviolet light. 2.2. Mass transfer measurement methods The analogy between heat and mass transfer has been widely used since the 1950s. The local HTCs are experimentally acquired by local mass transfer measurements. The analogy theory assumes that a constant concentration gradient in the boundary layer of a mass transfer process is analogous to the constant temperature gradient of a heat transfer process. Therefore, if local mass flux can be quantified, the local mass transfer coefficient is obtained. By verifying the boundary layer conditions and specifying the ratio between heat and mass transfer (Pr/Sc), the local HTC can be quantified. There are three techniques to quantify local mass transfer. The well-known naphthalene sublimation method, the ammonia absorption method, and the swollen polymer method. 2.2.1. Naphthalene sublimation method The naphthalene sublimation method is based on the analogy between heat and mass transfer, which has been widely used in air-side heat transfer research. Goldstein and Cho [9] summarized the general procedure of this method. Naphthalene is coated on the solid surface of interest; the initial weight and surface contour are measured. Next, the sample is exposed to the moving air flow in which the naphthalene coating sublimates and dissipates due to the air flow. Finally, weight and surface contour are measured again. Thereby, the time-averaged local mass transfer coefficient is acquired. According to Goldstein and Cho [9], this method can be utilized on many surface structures such as flat plates, cylinders, airfoils, cylinder arrays, pin-fin arrays, and cavities. The local HTCs acquired on the squared cylinder and circular cylinder by forced convection show a similar trend based on the comparison to results from other researchers. However, absolute values have more than 20% variation. According to Mendes [10], sample preparation is demanding because the surface must be glasslike smooth. Moreover, in order to avoid influencing the air flow, the depth change (Dd) of the

naphthalene before and after mass transfer needs to be minimized. The accuracy of the depth change measurements also needs to be considered. If the naphthalene sublimation depth is too small (<25 mm), the accuracy of the depth measurement (4 mm resolution) will be poor. Otherwise, if the depth change is too large (>200 mm), surface deformation will impact the flow characteristics. They also emphasized that a 1 °C wall temperature difference can cause a 10.1% concentration variation of naphthalene vapor. Since the mass concentration of naphthalene vapor is very sensitive to temperature, a constant temperature needs to be maintained during the mass transfer experiment. This means that if both heat transfer and mass transfer occur at the same time, this method is not reliable. Furthermore, natural sublimation of naphthalene needs to be evaluated and considered for specific experimental conditions. The naphthalene sublimation method is also utilized on some more complicated geometries. DeJong and Jacobi [11] employed this method to quantitatively acquire local and row-by-row HTCs of a seven-row parallel-plate array. Compared to real fin arrays, their tested samples are scaled-up for about ten times to W25.4 mm  H152.4 mm  T3.2 mm. Their work provided insight on detailed heat transfer distribution and how vortex shedding interacts with heat transfer. The averaged results showed good agreement with previously published data by Shah and London [12]. Although the naphthalene sublimation method is employed to quantify local HTC in a wide range of applications, the disadvantages such as safety concerns and the complexity of sample preparation make this method unfavorable. Furthermore, there is no functional method to apply a thin, uniform, and smooth naphthalene coating on complicated surfaces such as a real fin-andtube heat exchanger. It is also challenging to obtain consistent depth contour on non-flat surfaces.

2.2.2. Ammonia absorption method (AAM) The ammonia absorption method (AAM) also utilizes the analogy between heat and mass transfer. Compared to the naphthalene sublimation method, the direction of mass transfer is reversed. This method was first proposed by Kottke et al. [13] in 1977. A white test paper is soaked in the solution, which contains water, MnCl2, and H2O2. The sample to be measured is then covered with the wet test paper and loaded in a wind tunnel. Dilute ammonia

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gas (100 ppm) is injected and well mixed with air flow which flows across the surface. Therefore, ammonia will have a chemical reaction with the wet and acidic surface. The final product of the chemical reaction is MnO2, which is brownish in color and deposits on the original white paper. Subsequently, the paper is removed from the sample and dried by heating for color observation. The remission rate (intensity of color) and ammonia mass transfer are correlated by photometry. By applying the analogy between heat and mass transfer, the local HTC can be quantified. However, several issues made this method only suitable for mass transfer visualization instead of quantification. As it is difficult to have the wet paper tightly attached to the sample surface, the boundary layer conditions are to be evaluated. Furthermore, the color boundaries will not be as sharp on the wet surface, which will cause an error in measurements. According to Kottke et al. [13], a fifth-order equation was used to correlate color change to the mass transfer. Due to the limitation of image processing techniques forty years ago, they only qualitatively showed the local mass transfer distribution with a photograph. No quantified local HTCs were reported. Sadeh et al. [14] developed a similar method with a color-change dry coating to measure an external flow through a cylinder. As water is necessary for the chemical reaction, the ammonium hydroxide solution is sprayed onto the cylinder surface. Similarly, they only did visualization of the separation angle instead of quantifying local HTC. Ahrend et al. [15] employed this method to compare local HTCs of two heat exchanger designs. A round tube structure and an inclined flat tube structure were measured and compared, which provided insight into the better heat transfer approach. However, they still used the soaked test paper, which is not possible to be applied to complicated geometries such as louvered fins. Moreover, some boundary layer concerns such as the thickness and roughness of the filter paper, the attachment issue, and the poor resolution of the color contour still exist. Therefore, the accuracy of this method is unknown. 2.2.3. Swollen polymer method The swollen polymer is another type of coating material for local mass transfer measurement. According to Saluja et al. [16], compared to naphthalene coating, polymer coated samples can be reused. The working principle is to have the polymer coated sample soaked in the swollen agent first. The thickness of the coating will increase, after which an initial surface contour measurement is taken. When the sample is exposed to the air flow, the solution evaporates, and the coating thickness decreases. By measuring the thickness change, the local mass transfer distribution is acquired. However, the difficulty in quantifying the saturated vapor pressure of the swollen agent, which will cause about ±30% uncertainty of the local mass transfer coefficient measurements. Lampard and Hay [17] applied vulcanized silicone elastomer as the coating material and ethyl salicylate as the swelling agent; they measured local HTC on a discrete hole and slot injection film cooling geometry. However, the polymer coating they applied was 500 mm thick, which can have a non-negligible influence on the air flow. Therefore, this method is only suitable to observe local HTCs on simple geometries. Only several publications in two research groups have been found which applied the swollen polymer method to measure mass transfer: Saluja et al. [16], Lampard and Hay [17], Ammair et al. [18], Lampard and Hay [19], and Roberts et al. [20,21].

AAM. A color change coating and tracer gas combination is developed; the chemical interaction between the coating material and a tracer gas is the driving force of mass transfer. As shown in Fig. 2, a color change acidic coating material is attached to the heat transfer surface of interest. A dye is mixed with the coating material, which changes color at a certain pH level from yellow to blue. A tracer gas (dilute ammonia at ppm level) and air flow are well mixed prior to passing across the coated sample. As the coating material absorbs tracer gas and is neutralized, the color change can be observed on the surface. Therefore, convective mass transfer takes place as the tracer gas is transferred from the free stream flow to the coated surface. If the color change can be correlated with local mass flux and mass transfer coefficient, by applying the analogy, local HTC can be quantified. Since the coated surface is dry, and some amount of water vapor is necessary for the chemical reaction to take place, the humidity ratio of the air flow must be controlled. For the consistency of experiment, 26 °C, 60% relative humidity (RH), 50 ppm volumetric concentration of ammonia and air mixture, and 1 m/s flow velocity are set as the baseline condition for the mass transfer experiment. 3.2. Boundary layer assumptions and the analogy theory A schematic of the convection boundary layer for laminar flow over a flat plate with the infinite spanwise length is shown in Fig. 3. There is no temperature difference between the fluid and the plate, so the isothermal mass transfer is assumed. In the boundary layer, it is assumed that the streamwise velocity profile and the tracer concentration profile have a similar shape. The tracer concentration difference is caused by the chemical reaction between the coating material and the tracer gas. The chemical potential between the coating material and the tracer gas is strong enough that the tracer concentration at the wall is close to zero. The thin film coating is on top of the substrate, which represents the interactive wall. Ideally, the coated surface should be dry, thin, and smooth. Incompressible fluid, no-slip, and impermeable wall assumptions are well accepted for laminar flow over a solid plate. For the plate covered by the thin film coating, these assumptions are still valid, because the bulk flow cannot penetrate through the wall. Although the tracer gas is absorbed by the coating material, the permeability is still negligible, because 50 ppm ammonia is negligible compared to the bulk flow. The flow velocity in the streamwise direction and the coordinate direction normal to the plate are both zero at the interactive wall (U w = 0, v w = 0). For steady state, constant density, pressure, velocity, temperature and concentration of the bulk flow, if no-slip and impermeable wall assumptions are valid, the two-dimensional continuity equation shown in Eq. (1) is the same as the conventional heat transfer problem. T1 ; C1 and U are constant.

3. The proposed method (CTC) 3.1. Principle of the proposed method A new visualization method which employs the analogy of heat and mass transfer is developed. It applies a similar principle as the

Fig. 2. Principle of the proposed visualization method.

M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624

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a function of Reynolds number (Re) and Schmidt number (Sc). Pr is the ratio of viscous diffusivity (v ) and thermal diffusivity (a), while Sc is the ratio of viscous diffusion (v ) and mass diffusion (D). Pr and Sc are known properties of fluids at the known temperature and pressure. As a result, the ratio of Nu and Sh can be calculated, which is approximately equal to 1 when dilute ammonia is used as the tracer gas. The analogy theory is also explained by Kottke et al. [13] and Ahrend et al. [15], who employed the AAM. According to Poling et al. [22], for air at 26 °C, 60% RH, 101 kPa, Pr is 0.74, v is 1.61E05 m2/s; for air-ammonia mixture at the same temperature, RH and pressure, D is 2.14E05 m2/s, and Sc is 0.75. These parameters have been used to calculate local HTCs for the proposed method at the baseline condition. Fig. 3. Laminar momentum and mass transfer boundary conditions over a flat plate.

@u @ v þ ¼0 @x @y

ð1Þ

Meanwhile, if the coating is thin and the surface roughness is comparable to the metal base, the fluid velocity profile in the boundary layer will keep the same shape with and without the coating. Therefore, the momentum equation for the mass transfer will be the same as for the heat transfer shown in Eq. (2). It is worth mentioning that from this point of view, this new method offers an advantage over the naphthalene sublimation method because the coating will only change its color, while the sublimation of naphthalene will cause surface deformation.

u

@u @u @2u þv ¼l 2 @x @y @y

ð2Þ

After checking continuity and momentum equations, the next step is to apply the analogy between heat and mass transfer. As shown in Fig. 3, the tracer concentration in the bulk flow is 50 ppm. Meanwhile, the tracer concentration at the wall is approximately 0 ppm. This constant concentration difference is analogous to the constant temperature difference in the heat transfer. If the mass flux is measured, which is analogous to heat flux, the heat transfer coefficient is obtained. The detailed analogy equations are shown in Table 1, it can be seen that the format of heat transfer equations and mass transfer equations are similar. The heat transfer Nusselt number (Nu) is a function of Reynolds number (Re) and Prandtl number (Pr), as the mass transfer Sherwood number (Sh) is

Table 1 Analogy between convective heat and mass transfer Bejan [23]. Convection Heat Transfer

Convection Mass Transfer

Fourier’s law of heat conduction; q00 ¼ k  @T @y

Fick’s law of mass diffusion

q00 is heat flux, k is fluid conductivity Newton’s law of cooling; q00 ¼ hðT w  T 1 Þ

ð@T=@yÞ h ¼ k ðT w T 1 Þ

(3)

(5) (7)

h is convection heat transfer coefficient Energy balance, first law; @T u @T @x þ v @y ¼ a



@2 T @y2



Nu ¼ Pr ¼ va Nuloc ¼ f ðReloc ; Pr; xÞ Blasius solution, laminar flow, flat plate; hx k

Nuloc ¼ 0:332  Pr 1=3 Reloc 1=2 (Pr > 0.5)

(9) (11) (12) (15)

(17)

m00 ¼ D  @C @y

m00 is mass flux, D is diffusion coefficient Convective mass transfer; m00 ¼ hm ðC 1  C w Þ hm ¼ Dð@C=@yÞ ðC w C 1 Þ hm is convection mass transfer coefficient Mass conservation for the tracer gas;  2  @C @ C u @C @x þ v @y ¼ D @y2

(4)

(6) (8)

(10)

Sh ¼ Sc ¼ Dv Shloc ¼ f ðReloc ; Sc; xÞ Blasius solution, laminar flow, flat plate;

(13)

Shloc ¼ 0:332  Sc1=3 Reloc 1=2 (Sc  0.5)

(18)

hm x D

(14) (16)


1=3
1=3 Shloc Sc 0:75 ¼ ¼ ðLeÞ1=3 ¼  1:00 Pr 0:74 Nuloc

ð19Þ

Nuloc  Shloc

ð20Þ

3.3. Theoretical calculation for the baseline test condition For the baseline experimental condition, a W50 mm  L50 mm  T0.5 mm aluminum plate (Al 3003) will be used as the base metal. The local Re number can be calculated by Eq. (21). The characteristic length of the local Re number is at the location  in the streamwise direction. If the Re number is smaller than 5  105, the laminar flow condition is fulfilled for the external flow through a flat plate. For the baseline condition, ReL is 2950 when x is 50 mm (total length). Therefore, the local heat transfer coefficient (hloc ) and the mass transfer coefficient (hmloc ) can be acquired through Eqs. (17) and (18) respectively. Fig. 4 shows the span-averaged local HTCs and Nu number for this plate according to the Blasius solution.

Reloc ¼

Ux

v

ð21Þ

4. Experimental apparatus and procedures 4.1. Facility for mass transfer experiments 4.1.1. Description of wind tunnel A suction type, open-loop wind tunnel is designed and built for the mass transfer experiments, as shown in Fig. 5. The design followed the principles described by Barlow et al. [24] and ASHRAE Standard 41.2-1987 (RA 92) [25]. The size of the test section is W120 mm  H100 mm  L400 mm, which is to test small samples such as flat plates, single fins, and fin arrays. The flow velocity across the test section can be adjusted from 0 to 3.5 m/s. The air flow, water vapor (1), and anhydrous ammonia tracer gas (2) are injected at the entrance. The test section is made of transparent polycarbonate sheets for creating visual access required for the new test method. A webcam (7) is installed for image capturing. An electrochemical gas sensor (8) is connected to the test section to obtain real-time volumetric ammonia concentration measurements. ASHRAE nozzles (11) are used to measure flow rate and velocity across the test section. Downstream of the nozzle, a relative humidity sensor (12) is employed to track humidity of the flow. Moreover, the temperature distribution is measured by six Type-T thermocouples at different locations in the wind tunnel. The pressure is measured by piezoelectric pressure transducers. The cross-sectional flow velocity is measured with a calibrated hot wire anemometer. Local velocity data acquired from the anemometer generally agree well with the velocity measured by the nozzle. The cross-sectional flow velocity variation is within

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Fig. 4. Blasius solution for the baseline condition (a) Local HTC; (b) Local Nu & Sh.

1 5 9 13

water vapor and air 2 contraction 7.5:1 6 diffuser 10 blower

ammonia inlets test section honeycomb

3 7 11

air mixer webcam nozzle

4 8 12

honeycomb ammonia sensor humidity sensor

Fig. 5. Schematic diagram of the wind tunnel.

1%. The webcam parameters are set with MATLAB R2018a image acquisition toolbox to make sure the settings are constant during the mass transfer process. A 25 W LED white panel light is used to have stable illumination. The LED panel is located on the top of the sample, which is about 40 to 120 mm away from the color change surface. 4.1.2. Ammonia concentration control system As ammonia gas is mixed with the flowing air to obtain the desired concentration of 50 ppm, a system to achieve precise ammonia flow control is designed and shown in Fig. 6. A cylinder which contains anhydrous ammonia is immersed into a thermal bath with temperature control. A 0.038 mm micro-orifice is used, and by adjusting the orifice inlet gas pressure, the ammonia mass flow rate can be controlled. The ammonia vapor pressure is controlled through the temperature of the thermal bath. A nineoutlet distributor has been designed and installed at the entrance of the wind tunnel. The ammonia concentration is measured by an electrochemical sensor. A sampling tube is connected between the sensor and the air flow. By adjusting the location of the sampling tube, the concentration at different positions can be evaluated. As shown in Fig. 7, 50 ppm volumetric ammonia concentration is acquired when the orifice inlet pressure is 190 kPa. The concentration variation is about ±10%, which is consistent with the sensor reading uncertainty provided by the

sensor supplier. Therefore, the control of tracer concentration is achieved. 4.2. Image processing and local mass transfer quantification procedure An image processing procedure is developed and shown in Fig. 8. The first step is to have the sample coated with the color change material according to the developed coating procedure. Afterwards the maximum mass of ammonia absorption (M max ) on the entire coated surface can be calculated. Then the sample is exposed to the tracer gas in the wind tunnel for the mass transfer experiments. The wind tunnel is prepared to have a constant temperature, pressure, humidity ratio, tracer concentration, and flow velocity at the baseline condition mentioned in Section 3.1. The coated surface changes color from yellow to blue while the tracer gas is reacting with the coating material. MATLAB R2018a is employed for image processing to quantify the ratio of color change (
Þ at each pixel at a certain time (t) by hue analysis. Later, the correlation between the color change to the mass transfer coefficient is decided by a calibration test. The hue values show a linear correlation to the mass transfer coefficient. The coefficient of this linear correlation is called color change factor S. It has been verified that S is a constant at the baseline condition. Finally, by applying the analogy between heat and mass transfer, local heat transfer coefficient is quantified.

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Fig. 6. Ammonia concentration control system.

Fig. 7. Ammonia concentration control via orifice inlet pressure.

of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization. The titrant was made by mixing 10 g coating solution with 100 g pure water. An electric pH sensor and a magnetic stirrer have been put into the mixture. Then standard NaOH solution (1 mol/L) was added dropwise into the mixture until pH reaches 7 (the original pH of the titrant is 2). Therefore, the hydrogen protons (H+) of the coating solution are represented by the quantity of NaOH (OH) being used. This represents the capacity of the coating solution in terms of maximum ammonia absorption. Thus, the chemical relation between ammonia gas absorption and the solution is obtained as 1.49 mg/g. Then, 0.5 g coating solution is measured by a scale with 0.01 g precision level and coated to the 50  50 mm flat plate. The maximum mass of ammonia gas absorption (M max ) is obtained which corresponds to 0.745 mg on the samples for the baseline condition experiments.

4.2.1. Maximum ammonia absorption The coating material (AMS 67 manufactured by Serionix, Inc.) is diluted with ethanol by mass at a ratio of 1:3, to obtain a 25% solution. By conducting a titration experiment, the number of hydrogen protons (Hþ ) can be determined. Titration is a slow addition

4.2.2. Quantification of color change Referring to the TLC method, hue is used to describe color. Cantrell et al. [4] explain the mechanism between hue and color wavelength. The reason for using hue instead of RGB to represent color is explained in detail, and the process of converting RGB

Fig. 8. Procedure for acquiring local HTC.

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information to HSI is provided [4]. According to Ellinger [26], the distinctive color variation noted in the spectrum is known as hue, which is dependent on the wavelength of light being reflected or produced. In the Munsell color system, hue, value, and chroma are used to quantify and describe a color. Value specifies the degree of lightness or darkness. Chroma or colorfulness indicates the quantification of brightness or dullness. The term saturation represents colorfulness of an area judged in proportion to its brightness, which is calculated as the ratio of chroma and lightness. There are several standard systems to describe and quantify color according to different application purposes, such as HSV (hue, saturation and value) and HSL (hue, saturation and lightness), which are often used for digital computer systems. RGB (red, green & blue) is most commonly used by digital cameras. The standard and definition of different color spaces are explained by Fernandez-Seara et al. [27]. The equations to quantify a specific parameter and conversion between different color spaces are also provided [4,27]. The images taken from a digital camera or a webcam are usually described in RGB. If an image is opened in MATLAB, the RGB value of each pixel can be shown. Calculating the hue value of each pixel is a simple process in MATLAB. The hue ranges from 0 to 1 and corresponds to the color’s position on a color wheel. As hue increases from 0 to 1, the color transitions from red to orange, yellow, green, cyan, blue, magenta, and finally back to red. First, the program converts 0 to 255 RGB values to 0 to 1 (absolute value/255). Depending on which RGB color channel is the maximum value, three different equations are used: if red is the maximum value, hue value is calculated by the first line of Eq. (22); if green is the maximum, hue value is calculated by the second line of Eq. (22); similarly, if blue is max, then hue value is obtained from the third line of Eq. (22). Therefore, the hue of each pixel of the images can be quantified. The ratio of color change (
) can be calculated by hue analysis as shown in Eq. (23). The subscript ‘‘now” represents hue value of current color, ‘‘i” represents hue value of initial yellow color, ‘‘fc” means hue of final blue color. Hue difference between current color and the initial yellow color represents the level of color change; while the hue difference between the final blue and the initial yellow represents the maximum color change. Therefore, the ratio of color change is measured. As shown in Fig. 9, the averaged hue value of the initial yellow color on the entire surface (huei ) is 0.1688, the averaged hue value of the final blue color on the entire surface (huefc ) is 0.5592. The averaged hue at time t on the entire surface (huenow ) is 0.2677. From Eq. (23), the ratio of color change (
) on the entire surface is 0.25.

hue ¼ ððG  BÞ=ðmax  minÞÞ þ 0Þ=6; if max ¼ R; if hue is less than 0; add 1 to hue

ð22Þ

ððB  RÞ=ðmax  minÞÞ þ 2Þ=6; if max ¼ G ððR  GÞ=ðmax  minÞÞ þ 4Þ=6; if max ¼ B


¼

huenow  huei huefc  huei

ð23Þ

4.2.3. The color change factor S The color change factor S represents the ratio of actual mass transfer and maximum mass transfer when the color changes from original yellow to final blue. It is related to the characteristics of the dye because the dye changes color at a certain pH-level threshold. It will also be influenced by the experimental conditions such as relative humidity and tracer gas concentration. Therefore, S can be decided by running a calibration experiment.

Fig. 9. Image at time t.

The calibration is based on curve fitting. First, a flat plate sample is prepared with a known weight of coating material. According to Section 4.2.1, the maximum ammonia absorption by the sample is 0.745 mg. The mass transfer experiment is conducted at the baseline condition (26 °C, 60% RH, 50 ppm volumetric concentration of ammonia and air mixture, and 1 m/s flow velocity). According to Section 4.2.2, the color change is quantified by hue analysis. Thus, maximum mass transfer at each time step can be determined. As shown in Table 2, actual mass transfer at each time step is calculated by multiplying the maximum mass transfer and the factor S. Finally, the local heat transfer coefficient is obtained by the experiment. The measured heat transfer coefficients (hc ) are compared with the theoretical Blasius solution. It is found that the measured heat transfer coefficients (hc ) and the Blasius solution agree best when the color change factor is 0.3. As shown in Fig. 10, different color change factors (S) are compared, the best curve fitting is obtained when S is 0.3. This means that when 30% of the coating reacts with ammonia gas, the color changes from yellow to blue. Thus, by multiplying S and e, the local mass transfer distribution on the coated surface is obtained. Mass transfer and color change are correlated locally. There are two important findings from the calibration test. The color change factor S is independent of time during the mass transfer experiment. In addition, the color change factor S has been found to have a constant value of 0.3 over a wide range of conditions. 4.2.4. Local heat transfer coefficient (HTC) As shown in Eq. (24), time-averaged mass flux m00 which is perpendicular to the coating surface is obtained. According to Eq. (6) in Table 1, the local mass transfer coefficient is the ratio of mass flux and concentration difference which can be calculated by Eq. (25). In Eq. (24), hmloc represents the local mass transfer coefficient;
is calculated through image processing from Eq. (23); S is obtained by calibration; Atot is the total surface area of the sample; DC is a known parameter according to the previous assumptions and measurements. Therefore, the local mass transfer coefficient is a function of time. As shown in Table 1, the local Sh number can be calculated by Eq. (13). The analogy between heat and mass transfer is shown in Eq. (20). Therefore, local Sh number is equal to the local Nu number. For the heat transfer, if the local Nu number is obtained, the local heat transfer coefficient (hloc ) can be calculated from Eq. (11). Thus, the relation between the hloc and hmloc is determined as shown in Eq. (26). Eq. (27) shows how hloc is quantified through mass transfer measurements. Thermal conductivity and diffusion coefficient D of ammonia-in-air are known properties and their values are given in Section 3.2. Therefore, the expression in the parenthesis can be treated as a constant and the local HTC becomes a function of time.

9

M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624 Table 2 Calibration on a flat plate. Time [min]

Avg hue [–]

Averaged ratio of change e [–]

Maximum mass transfer [mg]

Color change factor S [–]

Actual mass transfer [mg]

Measured hc [W/(m2 K)]

Blasius solution hc [W/(m2 K)]

0 1 2 3 4 5 6 7

0.1559 0.1700 0.1986 0.2580 0.3342 0.4244 0.4957 0.5110

0.00 0.04 0.12 0.29 0.50 0.76 0.96 1.00

0.000 0.030 0.089 0.216 0.373 0.566 0.715 0.745

0.3

0.00 0.01 0.03 0.06 0.11 0.16 0.21 0.22

– 51 25 17 13 10 8 7

1 186 24 16 12 10 9 8

Fig. 10. Calibration of color change factor S through different local hc experiments”.


 S  Mmax

ð27Þ

corrosion effects need to be considered. If a solution is highly corrosive, it reacts with the metal and changes its color as soon as it has been dropped on the coupon. Some solutions show spot corrosion within several days. The corrosion observation lasts for three days. The last step is to observe the color change. The coated samples are exposed to the ammonia and air mixture at the baseline condition. The coupon changes color from yellow to blue from 7 to 10 min. The color changes from the leading edge to the inner locations along the flow direction. Finally, solution AMS 67, a commercially available product, has been identified to meet all the requirements and ready for the mass transfer measurements.

5.1.1. Coating formulation development Different coating solutions are developed and evaluated on aluminum alloy Al 1100. The criteria include solution stability, metal corrosion, coating quality, and color change characteristics. The first step is to evaluate the uniformity and stability of the solution. Different formulations are prepared and diluted with ethanol to obtain 25% solution by mass. No chemical reaction is expected in ethanol for at least one week. The number of hydrogen protons (Hþ ) are measured during the course of one week. The second step is to check the feasibility of coating the solution on the metal surfaces. Amounts of 0.5 g of the diluted solutions are attached to the aluminum plates by dropping method. The solution should spread well on the sample surfaces. The coated surface should be smooth, the coating thickness and color should be uniform. Meanwhile, the

5.1.2. Applicable base materials Coating solution AMS 67 is coated on different base materials to evaluate potential compatibility problems. As the coating material is only expected to react with ammonia gas to quantify mass transfer, this evaluation mainly focuses on whether the solution will react with different metals and how to minimize corrosion. Table 2 shows different base materials that have been coated. Due to the characteristic of the coating material, the reaction can be observed by color change. The original coating solution is blue, and after being coated and dried, it turns yellow on the base material. If the solution reacts with metals, blue spots will appear on the yellow surface within several minutes to several hours. As shown in Table 3, the coating can be applied without any issues to most materials, which means this method has the potential to be used on many different types of heat transfer surfaces. As Al 3003 is mostly used for heat exchangers, future experiments will focus on this material. For carbon steel and zinc-galvanized carbon steel, a protective base paint is necessary before applying the coating.

m00 ¼

hmloc ¼

hloc ¼

m00 DC

ð25Þ

hmloc  k D


hloc ¼

ð24Þ

t  Atot

ð26Þ 


 S  Mmax  k Atot  DC  D



1 t

5. Experimental results 5.1. Coating evaluations

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M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624

5.1.3. Sample storage requirement As both solution and base materials are selected, the effect of sample aging needs to be investigated. Al 3003 sheets are coated with AMS 67 at different concentrations and are exposed in environments having different relative humidity. The detailed test conditions and results are shown in Table 4. It can be concluded that increased humidity reduces sample stability. The reaction between the coating material and metal base happens in highly humid (RH > 60%) environments in several hours. Therefore, the coated samples should be stored in a relatively dry environment (RH < 40%). Since the sample metal sheets are small, a sealed container is used to store the coated samples, and silicon desiccant is used to maintain a dry condition. Temperature impact is also tested by exposing the container with the coated samples and desiccant in it to 50 to 60 °C for two days. No detrimental effects caused by the elevated ambient temperature were observed. Therefore, it appears that there is no need to control the storage temperature.

d¼

5.2. Coating thickness quantification 5.2.1. Averaged coating thickness by weight measurement Three different coating methods are evaluated. In addition to the drop and spray coating, a spin coating technique was also considered. For drop coating, it is found that the mobility of the solution at 50% mass concentration is insufficient to ensure a uniform coating. For spray coating, the solution at 50% mass concentration is too viscous and resulted in blocking the spray nozzle. Therefore, the solution at 25% mass concentration has been selected for future experiments. Spin coating is also tried. It is only applicable to flat plate samples and was found to be difficult to achieve sufficient uniformity. Thus, the spin coating will not be considered for future experiments.

Table 3 Coating evaluation of different materials. Base material

Main element

Application

AMS 67 applicable?

Al 1100

Al

Yes

Al Al Al Al

Al & Mn Al & Mg Al & Mg Al & Mg & Si & Cu Cu Cu & Zn

Pure, cold metalworking Heat exchanger Corrosion resistant Corrosion resistant Stronger, pipe, fittings Electric components Cylinders, fittings, decoration Corrosion resistant Machinery Anti-corrosion Containers, decoration Tanks, barrier, decoration

Yes

3003 5005 5052 6061

Cu 110 Cu 260 Stainless 304 Carbon steel Zinc-Galvanized Carbon Steel Plastic

Fe & Cr & Ni Fe & C Fe

Plastic

Acrylic

Polycarbonate

Table 5 shows the measured sample weights and calculated coating thickness. The results of drop coating and spray coating are both shown and compared. As the solution is a mixture of polymer, dye, acid, water and ethanol, more than 90% of the solution is liquid which will evaporate completely when heated (about 0.01 g solid in 0.5 g solution). The weight difference of the coated sample only represents the mass of the solid part of the coating. For drop coating, 0.5 g solution is used, while for spray coating, 1 g solution is used. The reason is that for the latter method, some solution will be lost during the spray process to the areas directly adjacent to the sample. A fine scale with 0.1 mg precision level and ±10% standard uncertainty is used to quantify the amount of solution that actually attached into the sample. The coating thicknesses are calculated by Eq. (28). The density of the coating material is 1200 kg/ m3, the original sample size is L25  W25 mm for the coating evaluation. The uncertainty of the sample size is ±1 mm in each direction. By combining the uncertainty of weight and length measurements, the uncertainty of coating thickness is ±0.5 mm.

Yes Yes Yes Yes Yes Yes Yes No No

Yes

m=q mass=density ¼ area A

ð28Þ

5.2.2. Coating uniformity evaluation through surface topography The thicknesses determined by weight measurements represent the averaged thickness, not taking into account the effects caused by non-uniform surface heights. A three-dimensional surface profiler is employed to evaluate the uniformity of the coated surface. Surface topography of coated samples is measured to evaluate surface roughness, coating thickness, and coating uniformity. The coated sample is shown in Fig. 11. Some locations are covered by a tape before being coated. Thus, by removing the tape after the coating process, the surface of both coated and uncoated areas can be scanned by the optical profiler. The sample was coated with the AMS67 solution with 25% mass concentration using a drop coating technique. The solution amount was 0.5 g. The surface scanning, which includes both coated and uncoated areas, is carried out. This method will result in some coating accumulating at the interface due to the surface tension of the liquid solution. Thus, the interfaces between the tape and the coated surfaces will not be selected when analyzing thickness. A sample is measured with the optical three-dimensional surface profiler and shown in Fig. 11. About 0.8 mm2 surface area is scanned. Fig. 12 shows three selected areas for height analysis within 0.8 mm2. Area 1 is selected in the coated region, area 2 is the entire scanned surface, and area 3 represents the uncoated metal base. The detailed data are shown in Table 6. The averaged surface measurement results are summarized in Table 7, for each method, ten samples are coated and measured. The base metal to which coating was applied is Al 3003 with a mean surface roughness of 0.4 mm. The mean roughness of the drop coated surfaces was 0.9 mm, and the mean roughness of the spray coated surfaces was 1.6 mm. Accordingly, mean coating thicknesses were found to be 5.8 and 5.5 mm. According to Goldstein and Cho [9], the sublimation depth of naphthalene can be as much as 200 mm, which also relies on the analogy between heat and mass

Table 4 Stability of the coated samples. Metal base

Al 3003 50  50 mm

Mass concentration of solution [%]

100% (0.5 g) 50% (0.5 g) 25% (0.5 g) 12% (0.5 g)

Temperature 25–28 °C RH < 40% Desiccant

RH 40–60% Office room

RH > 60% Humid lab

>1 month >1 month >1 month >2 weeks

>1 week >1 week >5 days >3 days

2 days 1 day <1 day <1 day

11

M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624 Table 5 Averaged coating thickness (AMS 67 with 25% mass concentration).

Drop 0.5 g Spray 1.0 g

Metal base [g]

Coated sample weight [g]

Weight difference [g]

Calculated thickness [mm]

Uncertainty [mm]

1.7786 1.8970 1.6528 1.9252 1.8548 1.8696

1.7870 1.9047 1.6606 1.9342 1.8630 1.8775

0.0084 0.0077 0.0078 0.0090 0.0082 0.0079

5.4 4.7 5.3 5.3 5.1 4.8

±0.5

Fig. 11. Coated sample for surface measurement.

transfer. The measured naphthalene sublimation depth by DeJong and Jacobi [11] ranged from 25 to 120 mm, and the averaged depth change is about 60 mm. In conclusion, it appears that the coating layer is sufficiently thin and smooth compared to the wellapplied naphthalene sublimation method and will therefore not influence the air flow characteristics across the test section. This

is an important assumption that has to be made for the boundary layer in order to apply the analogy between heat and mass transfer. In summary, according to the investigation on flat plate samples, drop coating was found to be the most suitable method. The application process is simple, and the results for found to be satisfactory. However, the coating height close to the edges of the sample is thicker than at inner locations due to the surface tension of the solution. The coating thickness at the edge location of a drop coated sample is shown in Fig. 13. It is shown that the coating thickness at the edge is about 20 mm, while the coating thickness at the inner locations is about 10 mm. The variation can be as large as two times. Spray coating delivers better uniformity but relies on more developed skills of the person applying the coating. Moreover, there is some variation in the amount of the solution that is lost to the areas adjacent to the sample (spray loss). However, for the small size samples (50  50 mm), this can be corrected by weight measurement or running a calibration mass transfer experiment. For the large size samples, the spray loss influence will be evaluated according to specific geometries and applications. Therefore, for flat plate samples, the drop coating technique of the AMS 67 solution with 25% mass concentration was selected as the baseline to evaluate the accuracy of mass transfer.

5.3. Mass transfer and color change observation A coated metal sheet is located at the center of the test section, as shown in Fig. 14. The mass transfer experiment is conducted at the baseline condition.

Fig. 12. Surface topography investigated with 3D optical profiler.

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Table 6 Height measurements of the selected area in Fig. 12.

Area 1 Area 2 Area 3

Average height [mm]

Max. height [mm]

Min. height [mm]

Max.-Min. [mm]

7.6032 7.1026 0.2309

25.9279 97.7396 3.6001

8.3488 41.0963 4.7014

34.2767 138.8359 8.3015

Table 7 Coated surface measurement. Metal base

Solution

Coating method

Roughness metal [mm]

Roughness Coated [mm]

Coating thickness [mm]

Al

AMS 67 25% solution by mass

Drop

Mean: 0.4 ± 0.1

Mean 0.9 ± 0.3 Mean 1.6 ± 0.5

Mean 5.8 3.0/+6.6 Mean 5.5 2.2/+2.8

3003

Spray

Fig. 13. Surface topography of drop coated sample at an edge location.

When the mass transfer experiment begins, the webcam starts to take a video to record the color change process. The color change happens as soon as the sample is exposed to ammonia gas, which starts from the leading edge of the sample and spreads to the inner locations in the streamwise direction. The color change at four minutes is shown in Fig. 15. The images at different times are shown in Fig. 16. As the interest is focused on the streamwise direction, only the outlined center location of the image is selected. As mentioned in Section 5.2.2 that the coating is thicker at the edges due to surface tension, it takes a longer time for the edges to show the color change. To eliminate this error, about 1 to 2 mm of the leading and trailing edges in the images are cut when applying the image analysis. The trailing edge took 10 min to change to the blue color. As Fig. 16 only shows images within 7 min, the original yellow color is still existing at the trailing edge.

Fig. 14. Mass transfer experiment on the flat plate.

5.4. Image processing and local HTC The detailed procedure of image processing and local HTC calculations were explained in Fig. 8. The sample shown in Fig. 15 is analyzed according to the newly developed procedure. The images at different times (1 min for each step) are extracted from the video, and the pixels of the entire sample (except for the 1 mm cut areas at four edges) surface is selected to calculate hue. There are two reasons to cut the edges for image processing. One is to

reduce errors caused by thicker coatings; the other is to reduce the edge effects in the spanwise direction. As explained, the maximum mass of ammonia gas absorption is obtained, which is 0.745 mg for the samples of the baseline experiment. The color change factor S is confirmed as 0.3 through the previously described calibration process. Hue of each pixel is acquired according to Eq. (22) with MATLAB R2018a. The ratio of color change is calculated according to Eq. (23). Local mass transfer coefficient is

M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624

13

location x is obtained by Eq. (29). Ablue represents the area of blue color and 0.05 m is the spanwise length of the sample.

x¼

Fig. 15. Sample color at four minutes.

acquired by Eq. (25). Thus, local HTC can be calculated from Eq. (27). Fig. 17 shows the two-dimensional distribution of HTC across the coated surface. The data shown in Fig. 17a is based on experimental measurements, while the graph shown in Fig. 17b is obtained by plotting the Blasius solution accordingly. In Fig. 17, 300  300 pixels is equivalent to L50 mm  W50 mm. 5.5. Span-averaged local HTC The span-averaged results are shown in Table 8. In Table 8, by assuming a uniform spanwise HTC, the equivalent streamwise

Ablue 0:05

ð29Þ

As shown in Fig. 18, the green curve represents the well-known Blasius solution, while the black dots represent the experimental data points of five individual mass transfer experiments at the baseline condition. The error bars show the uncertainty of measurements, which is 15% considering both Type A and Type B uncertainties according to GUM [28]. Type B uncertainty is calculated, referring to the methodology explained by Moffat [29]. The detailed calculation of the uncertainty is shown in the Appendix A. The dashed lines show ±20% deviations compared to the Blasius solution. The measurements of local HTC show good accuracy except for the leading and trailing edges.

5.6. The effect of sample aging As previously explained, the coating is sensitive to water vapor content of the surrounding air. The coated sample is proposed to be stored in a dry environment (RH < 40%). Therefore, the color change and mass transfer experiments are also observed to evaluate the influence of sample aging. The results are shown in Fig. 19.

Fig. 16. Mass transfer and color change at different times.

Fig. 17. Two-dimensional distribution of HTC (ReL = 2950 at x = 50 mm); (a) Measured local HTC; (b) HTCs from Blasius solution.

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M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624

Table 8 Span-averaged mass transfer of one tested sample. Time [min]

Avg hue [–]

Ablue [m2]

Averaged ratio of change

0 1 2 3 4 5 6 7

0.1559 0.1700 0.1986 0.2580 0.3342 0.4244 0.4957 0.5110

0.00E+00 9.93E05 3.01E04 7.19E04 1.26E03 1.89E03 2.39E03 2.50E03

0.00 0.04 0.12 0.29 0.50 0.76 0.96 1.00


[–]

M [mg]

Equivalent Color change Position x [mm]

hc [W/(m2 K)]

0.00 0.01 0.03 0.06 0.11 0.16 0.21 0.22

0.0 0.1 6.0 14.4 25.1 37.8 47.8 50.0

1 51 25 17 13 10 8 7

5.8. Method robustness evaluation

Fig. 18. Span-averaged local HTC at the base line condition (ReL = 2950 at x = 50 mm).

It was found that the samples show the same performance within 45 days. 5.7. Local HTC measurements at different flow velocities As the local HTC measured at the baseline condition has been confirmed, the mass transfer experiments are carried out at different flow velocities by fixing other settings. The temperature, humidity ratio, ammonia concentration is kept consistent at the baseline condition. Flow velocities from 0.6 m/s to 3.0 m/s have been tested, and ReL is ranging from 1934 to 9422. Therefore, all tests are within the range of laminar flow criterion (ReL < 5  105). As shown in Fig. 20, three repeated measurements have been taken for each flow velocity, and the results are compared with the Blasius solution accordingly. The results also show good agreement with the theoretical calculation. The tested range of flow velocity represents the most commonly used range of air flow velocities across heat exchangers used for air-conditioning applications.

Fig. 19. Local HTC at different sample aging at baseline condition.

The baseline condition is precisely controlled, and the robustness of the method is checked under various conditions. The air flow temperature is controlled by adjusting the room temperature. The mass transfer experiments have been conducted at a room temperature between 21 °C and 31 °C. All other conditions such as flow velocity, relative humidity, and ammonia concentration are kept the same as the baseline condition. The measured local HTCs show no difference compared to the baseline condition. The color change factor S is 0.3 at which the best curve fit is obtained, yielding smallest deviations from the Blasius solution. It means the chemical reaction, color change, and mass transfer rate are independent of temperature within the range of 21 to 31 °C. The analogy of heat and mass transfer explained in Section 3 (Nuloc  Shloc ) works within this temperature range. Moreover, the relative humidity of the air flow is also evaluated ranging from 40% to 85%. Other parameters such as air temperature, flow velocity, and ammonia concentration are kept the same as the baseline condition. It is found that between 50% and 85% relative humidity, the analogy between heat and mass transfer works and the color change factor is a constant having a value of 0.3. It was also found that when the relative humidity is lower than 50%, the color change takes 1 to 2 h compared to 7 to 10 min at 60% or higher relative humidity values. Therefore, when the relative humidity is below 50%, the color change factors S changes and the analogy between heat and mass transfer fails. Due to the accuracy of relative humidity control, it is recommended to keep it higher than 60%. Ammonia, concentrations in the range of 35 to 85 ppm volumetric have been tested and all other parameters are fixed. The color change factor remained at 0.3. The analogy between heat and mass transfer works. The actual ammonia concentration should be used while calculating mass and heat transfer coefficients in Eqs. (25) and (27). However, higher concentrations are not tested, because the sensor range is limited to 0 to 100 ppm.

Fig. 20. Local HTCs at different flow velocities.

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M. Che, S. Elbel / International Journal of Heat and Mass Transfer 144 (2019) 118624

The robustness evaluation shows that within these tested conditions, local HTCs can be obtained by the mass transfer experiment and the analogy explained in Section 4 is validated.

6. Conclusions A new method to quantify local air-side HTCs has been developed, and this paper describes preliminary results for air flow across a flat plate at laminar flow conditions. The following conclusions are obtained: (1) The method (CTC) is based on the analogy between heat and mass transfer, similar to the well-known naphthalene sublimation method. The analogy between mass transfer and heat transfer is clear in laminar boundary layer flow, and the impermeable-surface assumption is valid at low concentrations. Moreover, the uncertainty of measured local HTC is 15%. The measurements also show good agreement compared to the Blasius solution. (2) The method employs an acidic color change coating material and a low concentration ammonia tracer gas to quantify local mass transfer which utilizes a similar chemical relationship as AAM. However, the CTC method has several advantages. The application of the thin film coating reduces the influence of the air flow at the boundary layer compared to employing the wet paper sheet. Moreover, the developed image taking and processing technique can correlate color change to the mass transfer with high accuracy. Absolute local HTC values can be obtained. (3) The coating material is compatible with metals which are widely used for technical heat transfer surfaces, the coating solution and coated surfaces are stable for at least 45 days, which is promising for applications. (4) The surface topography shows that the roughness of the coated surface is within 2 mm, and thickness is within 10 mm, which is much smaller than the naphthalene sublimation depth (25 to 120 mm). The thin film coating has a negligible influence on the fluid pattern at the boundary layer. (5) Image acquisition and processing procedure which employs MATLAB R2018a is developed to observe the color change and correlate color change to mass transfer. It is possible to obtain color change and mass transfer of each pixel with good accuracy without employing expensive experimental devices. (6) The measured local HTC from 0.6 to 3.0 m/s (ReL = 1934 to 9422 at x = 50 mm) agrees well with Blasius solution on the flat plate. Moreover, the method robustness is evaluated under different flow conditions. While it has been demonstrated that the new method is capable of quantifying local air-side HTC for the flat plate, it is believed to be applicable to more complex heat transfer surfaces as well. Compared to other existing experimental methods, this new method has several advantages: the coating material is not expensive; a simple coating method is suitable to be used for different geometries and materials. This thin film dry coating is superior to other mass transfer methods with respect to the influence on the boundary layer. Moreover, a generic wind tunnel can be used for the mass transfer experiments; a simple digital camera such as a webcam can be employed for color observation. The image processing method is simple, robust, and is based on a widely accepted MATLAB toolbox. The most important is that the result of the flat plate sample shows good accuracy and consistency. Therefore, this method has the potential to be further developed to analyze more involved heat transfer problems.

Acknowledgements 1. The authors would like to thank the member companies of the Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign for their financial and technical support. 2. The authors would like to thank Creative Thermal Solutions, Inc. (CTS) for providing technical support and equipment. 3. The coating measurements were carried out in part at the Frederick Seitz Materials Research Laboratory Central Research Facilities located at the University of Illinois at UrbanaChampaign. 4. The coating solutions are developed by Serionix, Inc., located in Champaign, IL. 5. The authors would like to thank Dr. Neal Lawrence for proofreading the article and providing valuable comments. Funding Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign and its membership companies provide funding for this project. Declaration of Competing Interest The authors declare that there is no conflict of interest. Appendix A. Uncertainty analysis The principle of uncertainty analysis is according to the methodology suggested by GUM [28]. Both Type A and B uncertainties have been calculated separately and then combined, as shown in Table 9. According to Moffat [29], the uncertainty of local HTC is calculated from Eqs. (27) and (30). In Eq. (30), X i represents each independent variable, while Y hloc represents the uncertainty of local HTCs. Variations of each independent measurement has been considered with 95% confidence level according to the instructions in the GUM [28]. The variables and their standard uncertainties have been listed in Table 9. The total uncertainty of measurements is 15% by combing type A and B uncertainties.


hloc ¼

Y hloc ¼


 S  Mmax  k



Atot  DC  D

( X
@hloc i

@X i



1 t

ð27Þ

2 )1=2  Yi

ð30Þ

Table 9 Uncertainty of local HTC measurements. Variable

Source of uncertainty

Units

Standard uncertainty




Hue measurement Calibration factor Obtained from other measurements Reference data, environment variation Obtained from length measurements Sensor accuracy Reference data, environment variation Image processing method Type B at t = 120 s Type B at t = 240 s Type B at t = 480 s Overall Type B showed in percentages Type A (5 repeated measurements)

– – mg W/m K m2 ppm m2/s S W/m2 K

±0.01% ±2.5% ±0.05 ±0.0005 ±0.00007 ±2.5 1.07E06 ±1 ±3.5 ±1.7 ±0.9 13% 8%

S Mmax k Atot DC D t hloc

hloc hloc

W/m2 K W/m2 K

16

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