The effect of wetting characteristics, thermophysical properties, and roughness on spray-wall heat transfer in selective catalytic reduction systems

International Journal of Heat and Mass Transfer 152 (2020) 119554

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/hmt

The effect of wetting characteristics, thermophysical properties, and roughness on spray-wall heat transfer in selective catalytic reduction systems David Schweigert a,b,∗, Björn Damson a, Hartmut Lüders a, Peter Stephan c, Olaf Deutschmann b a

Robert Bosch GmbH, Advanced Engineering Exhaust Systems, Wernerstr. 51, 70469 Stuttgart, Germany Karlsruhe Institute of Technology (KIT), Institute for Chemical Technology and Polymer Chemistry, Kaiserstr. 12, 76128 Karlsruhe, Germany c TU Darmstadt, Institute for Technical Thermodynamics, Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany b

a r t i c l e

i n f o

Article history: Received 29 July 2019 Revised 4 February 2020 Accepted 19 February 2020

Keywords: urea water solution spray-wall interaction boiling hysteresis Leidenfrost temperature static contact angle surface effects

a b s t r a c t The effects of wettability, thermophysical properties, and roughness on spray-wall heat transfer in automotive selective catalytic reduction (SCR) systems are investigated experimentally. Using static contact angle measurements the wetting characteristics of ferritic and austenitic steel, iron, tungsten, silver, nickel, aluminum oxide, and technical ceramics (Shapal-M, Sialon) are analyzed. Details of the transient spray-wall interaction on the heated material samples (initial wall temperatures 120 ◦ C–400 ◦ C) are captured with a high-speed camera. The heat transfer from the wall to the impinging spray is calculated from transient temperature fields by using infrared thermography measurements of the plate’s rear side. Our results reveal that the thermal preconditioning of the plate has a significant influence on its wettability, which induces a hysteresis in the spray-wall heat transfer boiling curve. Higher contact angles and higher thermal effusivity have considerable influence on the Leidenfrost temperature. On the contrary, changes in roughness Rz show only a minor effect. © 2020 Elsevier Ltd. All rights reserved.

1. Introduction The appropriate thermochemical conversion of AdBlue (32.5 wt% urea water solution, UWS) into the desired reducing agent ammonia NH3 for NOx reduction in automotive selective catalytic reduction (SCR) systems is still a subject of intense research in scientific and industrial development [1–6]. During the injection of UWS into the exhaust pipe, spray-wall contact potentially results in wall film formation. This is often the starting point for the growth of solid urea by-products. The thermolysis of urea into NH3 and isocyanic acid potentially enables the polymerization of dissolved isocyanic acid within the wall film. At a sufficiently long residence time undesired solid urea decomposition products are formed [7–12]. To prevent deposit formation it is essential to understand the physiochemical aspects during spray-wall contact. Targeted design of the exhaust system’s wall properties according to the system requirements could be one step towards a better SCR performance and robustness against deposit formation. Sur-

∗ Corresponding author at: Robert Bosch GmbH, Advanced Engineering Exhaust Systems, Wernerstr. 51, 70469 Stuttgart, Germany. E-mail address: [email protected] (D. Schweigert).

https://doi.org/10.1016/j.ijheatmasstransfer.2020.119554 0017-9310/© 2020 Elsevier Ltd. All rights reserved.

face and material properties are believed to influence the involved physical processes significantly. Within the short period of time during droplet-wall contact, the hydrodynamics of the impinging droplets and the wall heat losses due to the evaporation of the deposited liquid on the wall are of great interest. In literature, several investigations are dedicated to the effect of surface and material properties on heat transfer characteristics. One important characteristic thermal quantity is the Leidenfrost point. It is the minimal temperature at which a liquid droplet initially levitates on a fully developed vapor cushion above hot walls [13,14]. Nagai and Nishio [15] measured the Leidenfrost temperature of sessile droplets on extremely smooth sapphire surfaces and stainless steel with standard roughness. The authors found that particularly the wettability and the thermal properties of the wall material affected the Leidenfrost point. Bernardin et al. [16] investigated droplet impingement on samples with different surface finishes (polished, particle blasted and rough sanded). It was found that roughness had little impact on the temperature of critical heat flux (maximum heat flux) whereas the Leidenfrost temperature significantly decreased with rising roughness. For the Leidenfrost temperature TL of UWS droplets the following influence of roughness

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Nomenclature b cp f hs m p Q Ra Rz t T Tf

λ ρ σ θ T f

thermal effusivity (J/(Km2 s0.5 )) specific heat capacity (J/(kg K)) evaporated mass fraction (-) evaporation enthalpy (J/g) mass (kg) pressure (bar) heat (J) arithmetic averaged roughness height (μm) maximum roughness height (μm) time (d, h, s, ms) temperature (◦ C, K) transition temperature (◦ C) thermal conductivity (W/(m K)) density (kg/m2 ) surface energy (mJ/m2 ) Static ontact angle (◦ ) transition width (K)

Subscripts 1 related to first measurement amb ambient avg average CHF critical heat flux evap evaporated exp exposed g gas inj injection last related to last three measurements l liquid L Leidenfrost max maximum min minimum oven related to oven s solid spray related to spray state1 ambient steady state (Fig. 12) state2 low temperature state (Fig. 12) state3 high temperature state (Fig. 12) uws related to urea water solution w wall water related to water wi initially at wall Acronyms ASS ambient steady state HTS high temperature state LTS low temperature state RT room temperature SCR selective catalytic reduction SOI start of injection UWS urea water solution

Rz was proposed by Steinbach [17]:

TL = 0.065 K /



 μm2 R2z + 587 K.

(1)

In the report of Avedisian and Koplik [18], the Leidenfrost temperature increased with rising porosity of the investigated alumina surfaces. Furthermore, the measured droplet evaporation times on alumina surfaces were up to two orders of magnitude shorter compared to those observed on steel surfaces at the same wall temperature. The authors supposed that the generated vapor escaped through the porous layer promoting direct liquid-wall contact.

The influence of various surface properties (roughness, wettability, porosity) on critical heat flux was investigated by O’Hanley et al. [19]. The maximum heat flux on the porous hydrophilic surfaces was higher than the maximum heat flux on the porous hydrophobic surfaces. Roughness showed no impact on critical heat flux. The studies of Kim et al. [20] were dedicated to the influence of the contact angle on heat transfer. Compared to the hydrophobic surface, the hydrophilic surface showed a higher heat flux and a shift to a higher Leidenfrost temperature. Schmid et al. [21] investigated the crystallization and the related heat release of sessile UWS droplets on surfaces with different wettabilities at subboiling temperatures. The authors found two different crystallization modes which arose depending on the surface energy and wall temperature. Kruse et al. [22] examined the effect of surface chemistry as well as micro- and nanostructuring of austenitic steel on the Leidenfrost temperature of impinging droplets. In addition to the capillary effects in porous structures, a reduction of the contact angle and the intermittent fluid-wall contact due to the surface roughness significantly increased the heat transfer. Most of the studies considered sessile droplets or single droplet impingement. Few authors studied surface effects on spray impingement. Structured surfaces and their influence on heat transfer during spray cooling was investigated by Sodtke and Stephan [23]. Using infrared thermography measurements, the length of the 3phase contact line of the wetted area was determined. This length could be directly related to the heat flux and was used for characterizing the structured surfaces. Structured surfaces, which led to a longer 3-phase contact line (solid, liquid, gas), consequently enhanced the heat transfer for a more efficient wall film evaporation. The influence of micro-structured surfaces on heat transfer was also investigated by Silk et al. [24] and Tran et al. [25]. Wettability is seemingly an important factor determining the heat transfer characteristics. Surface preconditioning, in turn, affects wettability as described in the following studies. Takeda et al. [26] measured contact angles on various vacuum treated metal oxides. All materials showed extremely small contact angles due to desorption processes. Subsequently, increasing the exposure time under ambient conditions led to a propagating adsorption of hydroxyl groups (OH), and a subsequent adsorption of carbon substances. Static contact angles rose until they reached a steady state after approximately five days on all of the oxide surfaces. Becker et al. [27] determined the thickness of adsorbed organic species layers and contact angles on steel and titanium surfaces. With increasing layer thickness, the contact angles rose. Moreover, the wettability of steel was strongly dependent on the airborne humidity according to Bernett and Zisman [28]. Forrest et al. [29] provided experimental evidence for contaminants from the atmosphere and their effect on the wetting characteristics of aluminum and zirconium surfaces. The degree of contamination was varied with different cleaning methods, e.g. chemical etching and solvent cleaning. A hydrocarbon overlay significantly increased the contact angles and decreased the critical heat flux. Time-dependent contact angle changes of steel from hydrophilic to hydrophobic were observed by Kim et al. [30]. The storage conditions were varied to determine their influence on the contact angles (storage in nitrogen, immersion in water, variation of moisture and oxygen level). The authors found that the prior presence of oxygen was the most pronounced factor determining the wettability of stainless steel. Many studies have shown the dependence of contact angles on the adsorbed species. The wetting characteristics vary according to the pre-treatment and the subsequent exposure under different conditions. However, none of these studies consider the effect of

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Fig. 1. Experimental setup for the measurement of contact angles via sessile droplet method.

surface and material properties on spray-wall interaction with urea water solution in SCR systems. This study examines the surface and material related effects on urea water solution spray-wall interaction, which, to the best of our knowledge, has not been addressed before. 2. Experimental setup and procedure 2.1. Contact angle measurements The sessile droplet method is used to investigate and analyze the wettability of ferritic and austenitic steel, iron (Fe), tungsten (W), silver (Ag), nickel (Ni), aluminum oxide (Al2 O3 ), a technical ceramic based on aluminum nitride (Shapal-M), and a ceramic based on silicon nitride, aluminum nitride and aluminum oxide (Sialon). Static contact angles are measured to characterize the wetting characteristics of the surfaces, even though the more elaborate method of determining dynamic contact angles might reveal additional information. The corresponding material properties are given in Table 1. Fig. 1 shows the experimental setup for the measurements of the static contact angle θ of water on various surfaces. Deionized water is used instead of urea water solution because no significant changes of the contact angles are expected, which is also stated in Schmid et al. [21]. Furthermore, the liquids show almost equal surface tensions (σwater = 0.072 N/m, σuws = 0.075 N/m [31]). A Nikon 70D camera with a Sigma 25– 105 mm F2.8 EX DG Macro HSM lens and spacer rings were used to observe the droplets. The droplets, with a volume of approximately 10 μl, are applied with a micro-dispenser and a syringe with an outer needle diameter of 0.47 mm. The droplets fall from a height of 5 mm due to the gravity forces overcoming adhesion of the droplet at the syringe tip. This guarantees a reproducible droplet application. The camera lens, backlight illumination and droplet contact surface are aligned. The light source is scattered by a frosted-glass ensuring a homogenous illumination. After thermal exposure of the sample, the contact angle measurements are subsequently performed after cooling the sample plates to room temperature. The method of a double elliptic fit by Andersen and Taboryski [35] is used to extract the droplet-surface contact angles via MATLAB. Contact angles below 10◦ are not given, because the image processing algorithm does not operate reliably below this value. 2.2. Experimental procedure In order to guarantee reproducibility, the contact angle measurements are repeated 25 times on each of the surfaces. The measurements within the test series are averaged arithmetically (θ avg ). In addition, the contact angle of the first measurement (θ 1 ) as well as the averaged contact angle of the last three measurements (θ last ) are analyzed. Furthermore, the spray-wall heat transfer is measured on each of the investigated surfaces. The heat transfer measurements are

Fig. 2. Experimental setup for visualisation of wall wetting and measurement of heat transfer during spray-wall interaction (Reprinted (adapted) from Schweigert et al. [36] with permission from Elsevier).

carried out at a test bench, which is described in detail in Schweigert et al. [36]. A brief description of the setup is given in the following paragraph. 2.3. Measurement of spray-wall heat transfer A commercial 3-hole injector (Robert Bosch GmbH, ETI 5.2-12) pointing downwards into the direction of gravity can inject either UWS or water. A schematic representation of the setup is given in Fig. 2. The injector doses intermittently (injection duration tinj = 5 ms, injection pressure pinj = 5 bar ), whereby only the first spray impingement onto the dry plate is investigated. A flat plate is oriented 45◦ to the injector axis at an axial distance of 48 mm between injector tip and plate. The spray impingement onto the front side of the plate is captured via a high-speed camera. The heat loss at spray impact is quantified at the rear of the plate by evaluating the transient temperature fields from infrared thermographic recordings. A heat balance enables the calculation of an evaporated mass fraction fevap based on the sprayinduced heat loss Qspray normalized by the required evaporation enthalpy hs and sensible heat (liquid heat capacity cp,l , temperature difference between room temperature and saturation temperature Tl of the injected mass minj ) [36]:





fevap = Qspray / minj hs + cp,l Tl



.

(2)

The measurement points in the instable and stable film boiling regime can be fitted by an analytical function [36]:



 f Twi − Tf fevap (Twi ) = max acot ; fmin , π Tf

(3)

with Twi as the initial wall temperature at the start of the injection, f describing the varying maximum mass fraction, Tf as the

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Fig. 5. Contact angles after preconditioning at different temperatures (texp = 300s, water on ferritic steel, measured at room temperature (RT)).

Fig. 3. Measurement of spray-induced heat loss Qspray and evaporated mass fraction fevap , corresponding pool boiling mechanisms (I: convective boiling, II: nucleate boiling, III: instable film boiling, IV: stable film boiling) and characteristic temperatures TL and TCHF , UWS on ferritic steel (Reprinted (adapted) from Schweigert et al. [36] with permission from Elsevier).

Fig. 4. Evolving contact angles in ambient steady state (ASS) for different surfaces.

transition point of the curve and the transition temperature within the instable film boiling regime respectively. The transition width is defined as Tf . The minimum mass fraction fmin describes the low evaporated mass fractions in the film boiling regime above the Leidenfrost temperature. A typical result of a heat transfer measurement of a urea water spray onto a ferritic steel wall and its parameterized representation is given in Fig. 3. 3. Results and discussion Table 2 shows the results of the contact angle measurements on various materials. The sample plates have been previously exposed to temperatures of Texp = 550 ◦ C for an exposure time of texp = 50 h h to ensure stable surface properties, e.g. a constant oxide layer in case of metals and steels. After the heating, the sample plates are stored under ambient conditions for a longer time (tamb = 7 d). The contact angles are measured after the ambient exposure, wherefore the values are denoted as ambient steady state (ASS). Once exposed to higher temperatures again, wetting behavior alters with a significant decrease in the static contact angle, represented as high temperature state (HTS). In Fig. 4 the extreme values of the static contact angle measurements of the investigated materials in ambient steady state are depicted. The wettability of aluminum oxide (Al2 O3 ) is significantly higher than for the silver surface. 3.1. Dependence of wettability on prior thermal preconditioning For a better understanding of the temperature relevant effects on wettability, the ferritic steel plate is exposed to different oven

Fig. 6. Evolving contact angles dependent on thermal preconditioning under various preconditioning temperatures Texp (texp = 300 s, oxidized ferritic steel, measured at room temperature (RT)).

temperatures. After the plate is preheated in an oven to different temperatures for texp = 300 s (excluding the needed time for heat-up), it is cooled down to room temperature for the subsequent measurements. Droplets are sequentially applied and static contact angles are measured. Initially applied droplets (θ 1 ) behave (depending on the preconditioning temperature) significantly differently from those applied at the end of the test series (θ last ) and are therefore shown separately in Table 2. The measurements are carried out after the sample surface reaches a steady state (related to the static contact angles), which is at least seven days after exposure to ambient conditions. The Figs. 5 and 6 show the effect of thermal exposure (varying preconditioning temperatures) on the evolving static contact angles. The error bars represent the standard deviations of the measured contact angles. First, a slight increase in the contact angles can be observed up to a wall temperature of 200 ◦ C. Above a temperature of 250 ◦ C the contact angles begin to decrease. Temperatures above Texp ≥ 450 ◦ C do not result in a further decrease of the contact angles. The literature mentioned above reveals that the change in wettability can be explained by ad- and desorption processes [27–30]. Besides the thermal exposure, the prior wetting with water considerably influences the evolving contact angles. Fig. 7 depicts the contact angle dependency on the preconditioning temperature and the number of contact angle measurements. The preconditioning temperature Texp varies between room temperature (RT) and 550 ◦ C. The contact angles of the initially applied droplets differ substantially from those applied afterwards. The transient evolution shown in Fig. 7 reveals that the contact angle converges to a short-term saturated state right after the application of a few droplets. The steady state of the static contact angle is only re-

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Fig. 7. Contact angle dependency on preconditioning temperature Texp /◦ and measurement number (texp = 300 s, water on ferritic steel, measured at room temperature (RT)).

stored after a long-term exposure under ambient conditions. We assume at least two adsorption mechanisms with different kinetics, which eventually influence the surface energy. Consequently, the heat transfer during spray impingement is not only influenced by the variation in wall temperature, but also by a hereby induced change in surface wettability. These effects can be explained by the hydrodynamics of the impinging droplets which are dependent on the wall temperature and the surface characteristics. Other authors [37,38] measured the temperature dependence of the contact angle of water droplets under pressurized conditions to prevent the water temperature from reaching the boiling point. A rising surface temperature led to a decrease in the measured contact angle. This was explained by increasing adhesion forces of the liquid to the solid surface due to the reduction of intermolecular forces between the water molecules [38]. Bernardin et al. [37] suggested that organic residues on the surface also contribute to changing contact angles. We assume that during the impingement of droplets at high wall temperatures both effects are significant (decreasing surface tension of the liquid [38] and desorption of contaminants resulting in lower surface energy of the solid [37]).

Fig. 8. Experimental procedures to measure heat loss during spray/wall interaction.

3.2. Hysteresis in heat transfer During the measurement of spray-wall heat transfer, a hysteresis effect is observed depending on the temperature history of the sample plate. Measurements from high to low temperatures (Procedure A) induce other results than measurements from low to high temperatures (Procedure B). Fig. 8a schematically shows the experimental procedure with two different temperature profiles: Start to heat the sample plate (I) and hold it at the excessive temperature (II), rapidly transport the sample plate into the test-cell (III), start the injection (SOI) after falling below the desired injection temperature (IVa), simultaneously start the measurements (IVb), clean the sample with deionized water (V) and repeat the procedure, beginning at (I), this time with a lower injection temperature (III). The procedure is repeated until the lowest temperature of interest (Tw ≥ 80 ◦ C) is reached. Several short injections are applied in intervals of 1 s, but only the first injection is used for the evaluation. The measured heat transfer during spray impact of water on ferritic steel is depicted in Fig. 9. The dashed curves represent bestfit approximations of the single measurement points by Eq (3) with the fitting parameters Tf , Tf , fmin and f. The total transferred heat varies significantly dependent on the order of the experimen-

Fig. 9. Hysteresis in heat transfer during spray impact (water on ferritic steel); LTS=low temperature state, HTS=high temperature state, ∗ repeated measurements.

tal procedure. During the measurements (IV) all the experimental conditions are the same, only the thermal history of the investigated sample plate varies according to the aforementioned procedures (Fig. 8). The measurements with procedure A (high to low temperatures, labeled HTS) show a heat loss which is three times larger than for procedure B (labeled LTS) at a given initial wall temperature of Twi = 200 ◦ C. Both branches of the curve can be repeated in subsequent measurements indicating that the change is not due to surface oxidation. Also the experimental scatter between the repeated measurements is low. A detailed discussion of the measurement uncertainties can be found in [36]. Fig. 10 shows the heat loss depending on the order of the test procedure. Hysteresis also occurs when measuring the heat loss during the injection of urea water solution. The qualitative behavior differs substantially from the measurements with water. The upper curve is obtained when measuring from high to low temper-

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Fig. 10. Hysteresis in heat transfer during spray impact (UWS on ferritic steel).

atures (procedure A). If the measurements are performed from low to high temperature (procedure B), the Leidenfrost temperature TL is slightly shifted to lower temperatures compared to the measurements following procedure A. Also the temperature of critical heat flux TCHF and the maximum of the evaporated mass fraction fevap decrease. The differences in the characteristic thermal quantities are remarkable considering that the measurements are performed under the same conditions and only the short-time history of the plate is varied. The visual differences of the evolving wall films between the two hysteresis states – namely high temperature (Fig. 11a) and low temperature state (Fig. 11b) – can be seen from visual high-speed recordings. In the low temperature state obviously more mass is expelled during spray impact. The enlarged view of Fig. 11b shows the reflections of the lighting on the single liquid ligaments. Compared to the seemingly thinner wall film shown in Fig. 11a, the images indicate changes in surface wettability. Desorption processes at higher temperatures are assumed to change the surface energy and largely alter the impingement characteristics of the injected droplets. This indication encouraged us to investigate the wetting characteristics of the different states (low and high temperature state) using static contact angle measurements. 3.3. Dependence of wettability on heat transfer To prove the dependency of heat transfer on wettability, heat loss and contact angle are assessed correspondingly. After determining the spray-wall heat loss at distinct wall temperatures via infrared thermography, the static contact angles are measured by the sessile droplet method. The results are given in Figs. 10 and 12. Single measurements from low to high temperatures are performed starting from Twi = 155 ◦ C, raising Twi to 185 ◦ C and 220 ◦ C, respectively. The injection is repeated multiple times to get the sample plate into the low temperature state. After the heat loss measurements, the contact angles are measured in state 1, 2, and 3 (as indicated in Fig. 10). State 1 is the ambient steady state before the preconditioning at higher temperatures in the oven. State 2 is the low temperature state which is reached when tracing the hysteresis via the previously described procedure. Thermal exposure of the sample plate at higher temperatures leads to state 3, with the largest possible heat transfer in the instable film boiling regime. The static contact angles are θstate1 = 70◦ , θstate2 = 79◦ and θ state3 < 10◦ as depicted exemplarily in Fig. 12a–c. Since the setup and the investigated material sample are the same for the heat transfer measurements shown in Fig. 10, the

changes in heat transfer are likely to be caused by the different wetting characteristics (ambient steady state, low temperature state, high temperature state). A model representation of the typical surface layers on metal surfaces for the various wetting states is given in Fig. 13. The indicated metal layer corresponds to ferritic steel, on which an oxide layer forms under thermal exposure. The adsorbate layers are shown in Fig. 13a. Hydroxyl groups or water monolayers are likely to adsorb on the surface after atmospheric exposure. Subsequently, further airborne contaminants are likely to gather above this layer. Once exposed to higher temperatures these species desorb and the non-contaminated oxide surface remains present (Fig. 13b). Table 3 shows the typical surface free energies of the potentially present surface layers, given by Castle [40]. Surface wettability is seemingly dependent on the type and amount of the adsorbed species. Therefore, the measured hysteresis in heat transfer can be explained by thermally induced desorption and subsequent adsorption during atmospheric exposure. This greatly affects the wettability of the surface, which was evidenced by the static contact angle measurements on the various wetting states. Due to the changes in wettability, the boiling mechanisms and the expelled mass ratios depend significantly on the experimental procedure. 3.4. Wettability on various surfaces and its effect on heat transfer Heat transfer characteristics are determined in ambient steady state and high temperature state for a selection of several different materials (austenitic steel, ferritic steel, iron, nickel, silver, tungsten, aluminum oxide, Sialon and Shapal-M) as well as two liquids (urea water solution and water), shown in Fig. 14. The transition temperature Tf is given depending on the wettability properties. Wettability is characterized by the static contact angle of water at room temperature in the form of cos θ , which is known to be proportional to the surface energy σ s,g between solid (s) and gas (g) as described by Young’s equation [41]. The initially applied droplets on some of the surfaces after thermal preconditioning tend to spread arbitrarily over the surface forming a thin film, wherefore θ 1 is assumed to be 0°, where applicable. The surface energy of the unveiled surface (free of contaminants) is comparably high and the method in this case not applicable for the calculation of surface energy, which is indicated as a hatched area in Fig. 14. The first five contact angle measurements after thermal preconditioning (T = 500 °C) are used to characterize the wettability in the high temperature state. The static contact angles in the ambient steady state are determined after the storage of the sample plates under ambient conditions. The last three contact angle measurements of the test series are used to define the ambient steady state, indicated by θ last in Table 2. The intermediate states cannot be measured by the used method. Straight lines are introduced as a visual aid in Fig. 14, although a linear dependency is not necessarily expected. A significant shift of the transition temperature Tf for all of the materials can be observed, which is likely to be caused by the changing wettability through thermally induced desorption. Based on the findings about wettability, the following conclusions can be drawn concerning the dependencies of spray-wall heat transfer and wetting properties: 1. Hydrophilic surfaces shift the characteristic temperatures, i.e. Leidenfrost temperature TL (as also evidenced by Kim et al. [20]) and temperature of critical heat flux TCHF to higher values (in comparison to hydrophobic surfaces). 2. At a fixed wall temperature between the Leidenfrost temperature TL and the temperature of critical heat flux TCHF a higher

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Fig. 11. Frame of high-speed recording at initial wall temperature Twi = 225 ◦ C and t = SOI + 12 ms on lapped ferritic steel sample with UWS.

Fig. 14. Transition temperature Tf dependent on wettability cos θ .

Fig. 12. Evolving contact angles dependent on temperature state correspondent to heat transfer measurements (water on ferritic steel).

mass fraction is evaporated on a hydrophilic surface. This is probably caused by a higher area-volume ratio of the wall film and a longer 3-phase contact line, as discussed in Sodtke and Stephan [23].

3. Dependent on the present wall temperature and the prior wetting of the wall, the amount of transferable heat is altered according to the evolving wetting characteristics of the surface. 4. Contact angles are strongly dependent on thermal preconditioning. At higher exposure temperatures Texp > 400 ◦ C (for sufficiently long exposure duration texp ) most of the investigated surfaces show an initial contact angle θ 1 < 10°. 5. Hydrodynamics (fraction of expelled mass) as well as thermal effects (evolving boiling regime) are altered by changing surface wettability.

Fig. 13. Model representation of surface layers on metal surfaces, adapted from Eirich et al. [39].

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D. Schweigert, B. Damson and H. Lüders et al. / International Journal of Heat and Mass Transfer 152 (2020) 119554 Table 1 Properties of investigated materials - wall thickness d, density ρ , heat capacity cp , thermal conductivity λ, thermal effusivity b and measured roughness Rz ,1 rough estimation based on AlN. Material

d/mm

ρ / × 103 kg/m3

cp /J/(kgK)

λ/W/(m K)

b/kJ/(s0.5 m 2 K)

Rz /μm

Ferr. steel [32] Aust. steel [33] Fe [33] W [33] Ag [33] Ni [33] Shapal-M [34] Al2 O3 [34] Sialon [34]

0.5 0.5 0.5 0.5 0.5 0.5 1.3 0.5 2.0

7.7 7.9 7.9 19.3 10.5 8.9 2.9 3.9 3.24

460 500 435 133 235 431 8001 875 665

25 15 84 182 427 91 90 30.5 20

9.4 7.7 17.0 21.6 32.5 19.0 11.2 10.2 6.56

1.7 1.6 2.1 5.8 1.6 2.0 5.4 2.7 1.3

Table 2 Measured roughness Rz and static contact angles (θ avg , θ 1 , θ last ) at room temperature on various surfaces under different preconditioning (ASS=ambient steady state, HTS=high temperature state). Contact angles

Fig. 15. Maximum evaporated mass fraction fevap,max , Leidenfrost temperature TL and transition temperature Tf dependent on thermal effusivity b of the materials given in Table 1 (high temperature state, UWS).

3.5. Thermal effusivity of various bulk materials and its effect on heat transfer The extracted characteristics from the heat transfer measurement of the materials given in Table 1 are shown depending on the thermal effusivity b of the bulk material (Fig. 15). The heat transfer measurements of the materials in the high temperature state are used. In this state the wettability properties are almost equal among the materials (Table 2, θ last ). This enables the consideration of other material properties without the governing effect of wetting characteristics. The single data points are arranged into groups of metals/steels and ceramics. The results are presented in Fig. 15, showing the influence of thermal effusivity b on the maximum evaporated mass fraction fevap,max within the regarded temperature range (Twi = 120 °C − − 450 °C), Leidenfrost temperature TL and transition temperature Tf . According to the investigated samples a tendency of decreasing characteristic values (fevap,max , TL and Tf ) with rising thermal effusivity can be observed. This holds for both metals/steels and ceramics, though a separate consideration of these material groups is needed to clearly see this tendency. However, the results have to be critically reviewed, as the sample quantity is rather small. Nevertheless, at first glance the influ-

Material

Rz / μm

Fer. steel

1.7

Aust. steel

2.0

Fe

2.1

W

5.8

Ag

1.6

Ni

2.0

Al2 O3

2.7

Shapal-M

5.4

Sialon

1.3

T state

θ avg /

ASS HTS ASS HTS ASS HTS ASS HTS ASS HTS ASS HTS ASS HTS ASS HTS ASS HTS

69.9 26.0 57.1 23.4 63.0 30.3 66.1 27.3 75.7 28.9 45.8 35.0 24.7 19.8 54.2 40.5 62.9 16.6

◦

θ 1 /◦

θ last /◦

69.8 10.8 56.7 < 10 49.1 < 10 58.7 < 10 83.9 21.4 38.9 < 10 17.6 < 10 52.8 19.1 65.6 < 10

69.4 28.7 59.4 23.4 68.2 30.3 65.6 27.3 77.0 31.6 53.7 37.9 30.8 24.3 55.2 53.8 60.8 30.9

Table 3 Free surface energy for various surfaces, adapted from Castle [40]. Surface

Surface free energy / mJm-2

organic hydrocarbons organic polymers water metal oxides metals

ca. 20 ca. 20 − 30 73 200 − 500 10 0 0 − 50 0 0

ence of thermal effusivity on spray-wall heat transfer may not be as significant as it is described in the established literature. 3.6. Dependence of heat transfer on roughness According to the literature discussed above [15–17,19,22], surface topography by means of varying roughness is often assumed to cause changes in heat transfer during droplet impingement. With the use of various surface finishes, surfaces with varying roughnesses are prepared, listed in Table 4 including their microscopic appearance, roughness characteristics (Rz , Ra ) and measured contact angles θ avg . The roughness profiles are determined via a surface profiler (MarSurf M300). After preparation, the surfaces are exposed to oven temperatures of Texp = 650 ◦ C for texp = 50 h to allow the formation of an oxide layer. The heat transfer at spray-wall impingement is measured by the method described in Section 2.3 following procedure A. The heat transfer during spray-wall impingement on surfaces with various roughnesses is depicted in Fig. 16. No consistent

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Table 4 Various surface treatments and their influence on surface appearance, roughness (Rz , Ra ) and contact angle θ avg in ambient steady state (ASS). a) Untreated/standard

b) Lapping

c) Glass bead blasting

R z / μm

1.7

1.5

7.1

R a / μm

0.19

0.18

0.86

θ avg /◦

69.9

60.3

63.4

(ASS)

d) Flat grinding (unidirectional) 0.9 () 5.1 (⊥) 0.13 () 0.81 (⊥) 53.6 () 57.3 (⊥)

e) Side grinder scrubbing 35.7 5.2 n/a

or more specifically without changing the surface morphology and roughness. 4. Conclusions

Fig. 16. Evaporated mass fraction dependent on wall temperature Twi for different roughnesses (UWS on various treated ferritic steel).

dependency can be derived concerning an increasing roughness and the evolving heat transfer characteristics (TCHF , QCHF , TL ). This might be due to the fact that the complex three-dimensionally shaped surface cannot be described adequately by a single value. Wetting characteristics are highly dependent on the surface morphology. The quantity Rz should be primarily understood as a rough indicator to characterize the roughness. The measured heat transfer characteristic of the glass bead blasted sample plate shows the highest difference compared to the results of the other samples with a significantly smaller heat loss in the region of instable film boiling. Harris and Beevers [42] showed that grit blasting changes the surface chemistry due to the introduction of foreign atoms. The surface energy on steel significantly rose by residues from abrasive media mainly due to the introduction of sodium (Na). Moreover, glass bead blasting is the only one among the investigated treatments without vastly removing the upper surface layer. The microscopic images are shown in Table 4 and reveal the microstructure of the sample plates. Contaminants, which are introduced by abrasive media, as well as the removal of the upper surface layer significantly interfere with the consideration of a changing surface topography. Therefore, the differences in the measured heat transfer are very likely not solely attributed to a changing surface topography, but also to the introduction of surface contaminants. Aluminum oxide or silicon oxide are assumed to be irreversibly introduced into the upper surface layer. Those cannot be removed by thermal preconditioning

In this study, the effect of wetting characteristics, thermal properties, and roughness on spray-wall heat transfer of urea water solution was analyzed. Using an infared thermography camera, the heat transfer during spray impact on various surfaces was investigated. The wetting characteristics of these surfaces were measured by the sessile droplet method. Wettability is one of the prevailing material surface properties affecting heat transfer during spray-wall impact. Even with the simplification of using the static contact angles (instead of dynamic contact angles), a clear correlation between the shift of the Leidenfrost temperature and the static contact angle was observed. With an increase in the static contact angles (more hydrophobic) the Leidenfrost temperature is shifted to lower temperatures. The wall temperature at critical heat flux as well as the absolute value of critical heat flux increased with the decrease of contact angles (more hydrophilic). Nevertheless, all thermally preconditioned materials show a hydrophilic behavior with contact angles of θ 1 < 20◦ at initial wetting (preconditioning temperature of Texp ≥ 450 ◦ C). Some materials return quickly to their ambient steady state upon contact with water or under ambient exposure, whereas steel shows persistent contact angles strongly dependent on thermal preconditioning. The influence of roughness on heat transfer characteristics does not follow a simple trend. Changing the surface energy and the underlying surface chemistry seems to have a more pronounced impact on the evolving heat transfer than the changing surface topography in the considered scale and shape. Investigations in literature about the influence of material properties onto heat transfer have been performed without a knowledge about the thermally induced change in wettability and the resulting hysteresis. The differences in the heat transfer caused by changing bulk materials are potentially hidden by the more pronounced effect of thermal preconditioning. This has to be taken into account especially when studying heat transfer phenomena in the instable film boiling regime. Seemingly rather small modifications of the wall surfaces (ad- and desorption) have a significant effect on heat transfer and thus, in our application, on the deposit formation in UWS-based SCR systems. Future studies could be dedicated to a deeper understanding of the effect of surface layer properties as well as the chemical characterization of the expected airborne surface contaminants. Correlating the data with dynamic contact angles (advancing and receding) might reveal additional details, since phase change

10

D. Schweigert, B. Damson and H. Lüders et al. / International Journal of Heat and Mass Transfer 152 (2020) 119554

phenomena and the impingement of droplets are highly dynamic. With this information, a more targeted development of impingement target coatings with reduced wall film formation could potentially be enabled. Thereby, an improved SCR system robustness against deposit formation may be achieved. Conflicts of Interest The authors declare that there is no conflict of interest. CRediT authorship contribution statement David Schweigert: Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Björn Damson: Methodology, Formal analysis, Writing - review & editing, Supervision. Hartmut Lüders: Conceptualization, Writing - review & editing, Supervision, Funding acquisition, Project administration. Peter Stephan: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Olaf Deutschmann: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Acknowledgement P. S. and O. D. acknowledge financial support by Deutsche Forschungsgemeinschaft (DFG) Collaborative Research Centre SFBTRR150.

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