Experimental study on circulatory flash speed of aqueous NaCl solution circulatory flash evaporation

Desalination 392 (2016) 74–84

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Experimental study on circulatory flash speed of aqueous NaCl solution circulatory flash evaporation Yu Wang, Longwen Yu, Yousen Zhang, Dan Zhang, Qingzhong Yang, Junjie Yan ⁎ State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, PR China

H I G H L I G H T S • • • • •

Circulatory flash speed was defined as average drop rate of NEF during the residence time. The effect of main experimental parameters on circulatory flash was clarified thoroughly. With the increase of circulatory flash speed, non-equilibrium fraction has a minimum value. Comparative study on flash speed between static and circulatory flash evaporation was conducted. Volumetric heat transfer coefficient linearly increased with circulatory flash speed.

a r t i c l e

i n f o

Article history: Received 29 January 2016 Received in revised form 12 April 2016 Accepted 15 April 2016 Available online xxxx Keywords: Circulatory flash evaporation Flash speed Residence time Volumetric heat transfer coefficient Aqueous NaCl solution

a b s t r a c t In present paper, a series of experiments on circulatory flash evaporation of 15% aqueous NaCl solution was carried out under various main experimental parameters. Circulatory flash speed, which represented the superheated energy consumed in unit time in flash chamber, was defined as average change rate of the nonequilibrium fraction during the residence time in flash chamber. It indicated the intensity of flash evaporation. The experiment results showed that circulatory flash speed quickly decreased to a lowest point, then increased monotonously with the increasing superheat and finally became flat. Influence of parameters, such as flow rate, initial height of water film and equilibrium pressure on circulatory flash speed was also investigated. As circulatory flash speed increased, NEF had a minimum value. At the same circulatory flash speed, NEF decreased when superheat degree and equilibrium pressure increased. And comparative study on flash speed between static and circulatory flash evaporation was also conducted. Both theoretical and experimental results of volumetric heat transfer coefficient showed a linear relationship with circulatory flash speed. However, the experimental values were greater than the calculated values due to the liquid droplets entrainment. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Flash evaporation defines a violent boiling phenomenon when the superheated liquid is exposed to an abrupt pressure drop below its saturation pressure. The evaporation obviously reduces the temperature of liquid and rapidly generates a quantity of flash steam. Flash evaporation can be divided into circulatory flash and static flash according to whether water film flows in horizontal direction or not. Flash evaporation phenomenon results in a temperature drop due to phase change. It has been extensively applied in many industry process such as the energy recycle [1], vacuum flash evaporation cooling [2], water desalination [3,4], food dehydration [5] and so on. Thus flash evaporation is paid more attentions from researchers worldwide. As basic form of flash evaporation, the static flash evaporation was researched firstly. Miyatake et al. [6,7] conducted experimental research ⁎ Corresponding author. E-mail address: [email protected] (J. Yan).

http://dx.doi.org/10.1016/j.desal.2016.04.013 0011-9164/© 2016 Elsevier B.V. All rights reserved.

on flash evaporation of pure water with equilibrium temperature from 40 to 80 °C and superheats from 3 to 5 K. The non-equilibrium temperature difference NETD and non-equilibrium fraction NEF of static flash evaporation were defined in their articles. NEF, which is a nondimensional indicator used to evaluate the degree of perfection for flash evaporation, is defined as NEF ðtÞ ¼

T l ðtÞ−T e : T l ð0Þ−T e

ð1Þ

And it was found that flash evaporation underwent two exponential decaying phases. In the first evaporation period, boiling phenomenon is very intense with many bubbles generated within the bulk of water film, and in the subsequent evaporation stage, boiling only occurs near the liquid surface. The relationship between maximum penetration depth and superheat, liquid level and initial temperature of water film was also investigated. Saury et al. [8,9] performed a set of experiments

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Nomenclature NEF non-equilibrium fraction CFS circulatory flash speed (s−1) NETD non-equilibrium temperature difference (K) L length of flash chamber (m) B width of flash chamber (m) H height of flash chamber (m) D diameter of flash chamber inlet and outlet (mm) Q flow rate (L h−1) equilibrium pressure (kPa) Pe initial height of water film (mm) H0 T temperature (°C) u horizontal velocity (m s−1) mass fraction fm specific heat at constant pressure (kJ kg−1 K−1) cp mass of flash steam (kg s−1) mfs r latent heat of vaporization (kJ kg−1) h volumetric heat transfer coefficient (kW m−3 K−1) R radius of vapor bubble (m) Greek symbols τ residence time (s) ΔT superheat degree (K) ρ density (kg m−3) σ interfacial tension (N m−1) Subscripts in inlet out outlet be brine sat saturation BPE boiling point elevation c circulatory flash evaporation e equilibrium fs flash steam 0 initial state

of water film flash evaporation with initial water height of 15 mm, superheats varied between 1 and 35 K, initial temperature between 30 and 75 °C. They proposed a correlation between the final evaporated mass and superheats. They also examined the influence of initial water height on maximal amplitude of flash evaporation. Kim [10] identified several critical points based on experiments of static flash evaporation of pure water. Gopalakrishna et al. [11] performed experiments with concentration of aqueous NaCl solution ranging from 0 to 3.5% and proposed an expression for evaluating flashed mass of pool flash. Static flash speed was defined as average drop rate of NEF during fast evaporation stage in Zhang et al.’s study [12,13]. The heat transfer characteristics and steam-carrying effect in static flash under various flash speeds were also studied. Fath [14] conducted simulation research on a 6-stage MSF system. The flash chamber effectiveness β was defined to technically evaluate the flash chamber performance. It was found that increasing brine superheat, flashing surface area, number of active nucleation sites and residence time can enhance flash chamber effectiveness. Yan et al. [15] carried out comparative research on heat and mass transfer characteristics between static and circulatory flash. Volumetric heat transfer coefficient was introduced to evaluate heat transfer intensity in these two kinds of flash. Jin [16,17] performed both experimental and numerical simulated work on the single-phase flow in flash chamber. Several bubble parameters such as size, velocity and growth rate were estimated and analyzed in their study. They found the flow rate and water level influence the single-phase flow patterns in flash chamber obviously. Our research team [18,19] conducted experimental

75

study on circulatory flash evaporation of both pure water and NaCl solution. The influence of main experimental parameters on NEF and heat transfer characteristics was also analyzed. The former study mainly concentrated on the static flash evaporation or the flow patterns inside flash chamber of MSF system. And former experiments of circulatory flash evaporation were performed of low horizontal velocity. Cycle working fluids were usually pure water and low concentration of aqueous NaCl solution. However, concentration of brine solution becomes quite higher in current salt chemical industry. NEF, one of the most widely used indicators of flash evaporation, is total criterion. But for circulatory flash evaporation, it can only weigh the rest superheated energy at outlet of flash chamber. However, flow rate and initial water level influence not only the turbulent of the water film but also the residence time. For example, higher flow rate corresponds to a stronger turbulence and shorter residence time. Strong turbulence can enhance flash evaporation, but short residence time leads to more rest superheated energy at outlet. So, the true influence of experimental conditions on the intensity of circulatory flash evaporation was not clarified thoroughly. Therefore, in present paper, circulatory flash evaporation at the higher horizontal speed of 15% NaCl solution was conducted. And circulatory flash speed was introduced to eliminate the influence of the residence time in flash chamber. Then, the influence of the main parameters on circulatory flash intensity was analyzed clearly. Moreover, comparison of flash speed in circulatory and static flash was carried out. Finally, the relationship between volumetric heat transfer coefficient and circulatory flash speed was examined. 2. Experimentation 2.1. Experimental facility Experimental facility for circulatory flash evaporation is shown schematically in Fig. 1(a). The experimental system consists of four circulation loops: a basic hydrothermal loop, a flash steam loop and two auxiliary condensing loops. The basic hydrothermal loop includes a circulating pump, an heat exchanger, two metal rotor flow meters with measurement range from 0 to 1600 L h−1, flash chamber and heat exchanger used to cool the flash water. The heat exchanger is composed of 30 groups of external electrical heaters with power of 90 kW. The flash chamber is a tank with rectangular transverse-section of 0.1 (length L) × 0.1 m (width B) and height H of 0.66 m. The pipe diameter D of inlet and outlet is 25 mm. The front and rear faces are made of glass for visualization study. The steam circulation loop is mainly composed of a steam condenser and a Coriolis mass flow meter with the full scale of 110 kg h−1 which is used to measure the mass of flash steam. The two auxiliary condensing loops are designed to ensure flash steam is condensed completely. The vacuum pump and two auxiliary loops are operated to found an environment under desired pressure during experiments. Pure water or NaCl solution is heated up to desired temperature and pumped into the flash chamber by the circulating pump. Flash evaporation takes place as soon as the working fluids enter flash chamber. Meanwhile, data collection system starts recording experimental data. The pressures are gauged through pressure transducers with measuring range of 0–0.2 MPa and accuracy of 0.25% in full scope. The temperatures are measured with a set of T-type sheathed thermocouples which with precision of 0.2 K. The arrangement of the thermocouples in flash chamber is displayed in Fig. 1(b). 2.2. Uncertainty analysis and repeatability Experiments conditions are listed in Table 1. The uncertainty analysis for the experimental system was performed with the Moffat method [20]. The result is stated in Table 2. Fig. 2 clarifies that the repeatability of the experiments. It shows the changing law of NEF with inlet superheat at a pressure of 12.3 kPa and

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Fig. 1. (a) Schematic of circulatory flash system. (b) Thermocouples arrangement in flash chamber.

initial water film level of 130 mm with a flow rate of 600 L h−1. The data of these two experiments under the same conditions aligned fairly well in the whole superheat range. Result suggests that the repeatability of the experimental apparatus is very good and data is reliable. 3. Results and discussion 3.1. Non-equilibrium fraction (NEF) For circulatory flash evaporation, NEF at outlet of flash chamber is the ratio between the rest superheat here and inlet superheat shown as Eq. (2). It is a non-dimensional indicator used to evaluate the degree of perfection for flash evaporation when circle fluids flow through flash chamber. A bigger NEF represents more rest superheat energy and more imperfect flash evaporation. The inlet and outlet temperature of fluids were measured in our experiments to calculate NEF. NEF ¼

T out −T be T in −T be

ð2Þ

The working fluids used in our experiments is aqueous NaCl solution with a 15% mass fraction. The boiling point elevation of NaCl solution is considered when the NEF is calculated in our experiments. T be ¼ T sat þ T BPE

pressure yields a smaller NEF at the outlet of flash chamber, which signifies the higher equilibrium pressure results in more intense boiling. The higher equilibrium pressure requires a greater inlet temperature at the same superheat degree. If inlet temperature of water increases, latent heat of vaporization reduces so that heat required to evaporate working fluids becomes less. So flashing is more likely to occur. Fig. 4(a) and (b) illustrates three series of NEF profiles at the outlet of flash chamber under the different flow rates Q. It is found that the NEF firstly decreased and then increased with increasing flow rate. Taking Fig. 4(a) for an example, while equilibrium pressure and initial water level are the same, NEF is the smallest under 800 L h−1 comparing with that under 600 and 1200 L h−1. The explanation is provided as follows. The elevation of flow rate can strengthen the turbulence intensity which can enhance flash evaporation. But for the same flash chamber, the increasing flow rate corresponds to shorter residence time. When flow rate is very small, although residence time is long, intensity of flash evaporation is so weak that NEF is big which manifests an imperfect flash. According to the above conclusion, when the flow rate is large enough, the residence time will be so short that the fluids flow through the chamber almost without evaporation. So, most of the sensible heat is brought away by working fluids. The value of NEF will be close to 1. The NEF variation at the outlet of flash chamber at different initial water level H0 while the other conditions were maintained constant is

ð3Þ

The variation of NEF with the pressure of flash chamber pe is presented in Fig. 3((a) and (b)). Result suggests that the higher equilibrium

Table 1 Experiment conditions. Parameter

Experimental range

H/m pe/kPa Q/L h−1 DT/K fm

0.10–0.22 7.9–31.2 600–1400 0–20 0.15

Table 2 Uncertainty analysis of circulatory flash evaporation. Parameters

Absolute uncertainty

Minimal measured value

Uncertainty

pe/kPa H/m T/°C B/m L/m Q/L h−1 fm τ DT NEF CFS

0.5 1 × 10−4 0.20 1 × 10−4 1 × 10−4 8.0 5 × 10−4 – – – –

7.4 0.1 40.35 0.1 0.1 600 0.15 – – – –

6.75 × 10−2 1 × 10−3 4.56 × 10−3 1 × 10−3 1 × 10−3 1.33 × 10−2 3.33 × 10−3 1.34 × 10−2 4.56 × 10−3 6.45 × 10−3 1.49 × 10−2

Y. Wang et al. / Desalination 392 (2016) 74–84

77

2σT be Rρv r

ð5Þ

T in −T be ≥

where R is the radius of vapor bubble, ρv is the vapor density and σ is interfacial tension. TBPE is the boiling point elevation. For small superheat degree, according to Eq. (5), due to the extra static pressure generated by water level, only sites near the top of water film is active. It can be seen in Fig. 6 that, boiling phenomenon only occurs near the free surface and is very weak. Superheat energy is carried away by water whereas the sensible heat can't be transferred into latent heat timely. When the superheat degree increases further, boiling occurs in the whole flash chamber. The upper part of the flash chamber is full of bubbles and liquid fragmentation. The boiling heat transfer effect is enhanced greatly. In consequent, the superheat liquid turns to steam quickly and consumes a lot of heat. The bigger superheat degree yields a smaller NEF. In summary, NEF is a total indicator represents the perfection of circulatory flash evaporation. But flow rate and initial water level influence not only the turbulence of the water film but also the residence time in flash chamber which influence flash evaporation counterproductively. It is difficult to analyze the true influence of the flow rate and initial water level on the intensity of circulatory flash evaporation.

Fig. 2. Repeatability of this experiment.

shown in Fig. 5(a) and (b). It can be seen that for present experimental range the higher initial height of water film attains greater NEF. The extra static pressure generated by the water film height can suppress the flash evaporation. The higher water level corresponds to a bigger static pressure at the bottom of water film. Take 15% NaCl solution with a temperature of 50 °C at height 0.2 m as an example, the extra static pressure at the bottom of water film is about 2.2 kPa. Thus, the saturation temperature at the bottom is 3.35 K higher than that at the top of water film when the pressure of flash chamber is 12.3 kPa and 5.01 °C when the pressure is 7.4 kPa. It can be seen that NEF firstly increases and then decreases as superheat degree increases. Combining the visual research and theories, the behavior of NEF can be explained as follows. The flashing phenomenon at different superheat degree is displayed in Fig. 6. And similar to pool boiling, a temperature difference between water temperature Tin and saturated temperature Tsat under equilibrium pressure for nucleation is necessary. The temperature difference for nucleation was stated as Eq. (4) by ref. [21]. For NaCl solution, boiling point elevation needs to be considered. Eq. (4) can be deduced as Eq. (5). T in −T sat ≥

2σT sat Rρv r

ð4Þ

3.2. Circulatory flash speed (CFS) In order to break away from the influence of residence time, the circulatory flash speed is introduced based on NEF and residence time in flash chamber to evaluate flashing intensity. Eq. (6) in Yan et al.'s study [15] is adopted to calculate residence time. The average horizontal velocity u in flash chamber can be calculated by Eq. (7). τ¼

L u

ð6Þ

u¼

1  10‐3 Q  3600 BH

ð7Þ

Residence time τ under different experimental conditions are calculated and presented in Table 3 according to Eq. (6). It suggests that the large flow rate and thin water film corresponds to short Residence time in depressurized environment. Residence time differs quite a bit under different experimental conditions. For example, residence time under flow rate of 1200 L h−1 with 100 mm water level is 3.0 s which lasts twice as long as that under flow rate of 600 L h−1 while other conditions are kept constant. Therefore it is necessary to adopt circulatory

Fig. 3. NEF versus superheat degree under different pressure of flash chamber.

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Fig. 4. NEF versus superheat degree under different flow rates.

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79

Fig. 5. NEF versus superheat degree under different initial heights of water film.

flash speed to eliminate the effect of residence time when the evaporating intensity is evaluated. Circulatory flash speed is defined as the ratio of NEF difference between the inlet and outlet to residence time of working fluids in flash chamber. The circulatory flash speed (CFS) can be calculated with Eq. (8). CFS ¼

NE F in −NE F out τ

ð8Þ

NEFin, which is the non-equilibrium fraction at the inlet of flash chamber, can be expressed as be NEFin =NE F in ¼ TT inin −T −T be : (9)

So it is obvious that NEFin identically equals 1. NEFout can be expressed as NE F out ¼

T out −T be : T in −T be

ð10Þ

So Eq. (8) can be written as Eq. (11) CFS ¼

1−NEF : τ

ð11Þ

The physical significance of circulatory flash speed is the superheat energy consumed during unit time in flash chamber. Circulatory flash speed is considered to evaluate the true intensity of flash evaporation. Fig. 7 clarifies the effect of equilibrium pressure pe on circulatory flash

speed CFS. Result suggests that the circulatory flash speed increases as the pressure of chamber rises up. Due to the same inlet volume flow rate and initial height of water film, residence time under various equilibrium pressures which corresponds different inlet temperature has no difference. With the increasing of superheat degree, NEF and circulatory flash speed CFS change with a contrary tendency which is consistent with the definition. So the explanation for the behavior of circulatory flash speed CFS is the same with NEF. Higher equilibrium pressure pe leads to greater inlet water temperature which has a smaller latent heat. This change of the working fluids can help to yield more evaporation cores. Flash evaporation is more intense at a higher pressure when the inlet superheat degree is the same. In Fig. 8, Circulatory flash speed versus superheat degree is plotted at various flow rates while pressure of flash chamber and initial height of water film is maintained constant. As shown in these figures, unlike the behavior of NEF, when the influence of residence time is removed, it is obtained that higher flow rate attains larger circulatory flash speed at the same degree of superheat due to the more intense turbulence in water film which indicates that enlarging flow rate can enhance the intensity of flashing. And Fig. 9 illustrates results acquired for the circulatory flash speed at various initial height of water film. For present flash chamber, a thicker initial water film corresponds to a smaller circulatory flash speed. The decrease of initial water film level can hasten circulatory flashing. The influence of residence time can be excluded when circulatory flash speed is used to evaluate the real flash intensity. And then the

Fig. 6. Circulatory flashing phenomenon at different superheats with flow rate of 1200 L h−1, equilibrium pressure of 12.3 kPa and water film level of 130 mm.

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Table 3 Residence time under various flow rate and water film height. H/mm

100 130 160 190

Qin/L h−1 600

800

1000

1200

1400

6.00 7.80 9.60 11.40

4.50 5.85 7.20 8.55

3.60 4.68 5.76 6.84

3.00 3.90 4.80 5.70

2.57 3.34 4.11 4.89

influence of flow rate Q and initial water level H0 on circulatory flash evaporation can be analyzed clearly. In our experiments, working fluids flow into the flash chamber from a submerged inlet through a ball valve. The changes of flow rate and initial water level lead to various average velocity of water film which affects agitation of water film pronouncedly. When the mixing is enhanced, nucleation is more likely to occur. So, severe agitation can enhance the flash evaporation and increase circulatory flash speed. The flow of water in flash chamber is a typical open channel flow. And thus Reynolds number (Re) and Froude number (Fr) are recruited to weigh the turbulence or agitation in water film. Re and Fr are found to be Re ¼

ρudw μ

where u is the average horizontal speed of working fluids in flash chamber can be calculated by Eq. (4), dw is the hydraulic diameter of water film. dw ¼

2H0 B ðH 0 þ BÞ

Re versus initial water film height H0 under several different flow rates Q and different inlet temperature of water is depicted in Fig. 10. It can be seen that, Re increases with increasing flow rate and decreasing initial water film height. Fr¼

u2in Dg

where uin is the velocity at inlet of flash chamber. Froude numbers (Fr) are 0.47, 1.31 and 2.56 that correspond to flow rates of 600, 1000 and

1400 L h−1 respectively. It is obvious that the higher flow rates yields a greater Fr. Re and Fr of water film differs a lot under various flow rates and initial water film height. Whereas the Re and Fr reflects the turbulence strength, the conclusion can be drawn that increasing flow rate Q and thinning initial water level H0 can enhance the turbulence intensity and improve mixing, thus tending to promote formation and growth of bubbles in flash chamber. So the behavior of circulatory flash speed CFS can be explained. Moreover, it is noteworthy that Re dwindles as temperature decreasing which suggests that turbulence intensity of water film is higher at a higher temperature. This changing law can also explain why CFS decreases with the decreasing of equilibrium pressure of flash chamber. Circulatory flash speed CFS versus superheat degree under different water film concentration is shown in Fig. 11. Results suggest that pure water yields a greater CFS when the other experimental conditions are kept constant. The water salinity can suppress boiling in circulatory flash evaporation. The different behavior of CFS between pure water and 15% NaCl solution can be explained with the difference of surface tension. Surface tension of NaCl solution is higher than pure water. According Eqs. (4)–(5), NaCl solution requires a higher temperature difference for bubble nucleation. Therefore, Circulatory flash speed of pure water circulatory flash evaporation is greater than that of NaCl solution. The experiment results showed that in all cases circulatory flash speed decreased to a lowest point, then increased monotonously with the increasing superheat and finally became flat with further increment of superheat degree. Circulatory flash speed CFS is defined as average change rate of the non-equilibrium fraction NEF during the residence time in flash chamber. It can be calculated with Eq. (6). Under the certain experimental condition, the residence time τ is the same. Hence, with the increasing of superheat degree, NEF and circulatory flash speed CFS change with a contrary tendency. As is shown in Section 3.1, with the increase of superheat degree, NEF increases initially and then decreases. The explanation of such trend also has been presented in Section 3.1. Figs. 12 and 13 displays the relationship between NEF and circulatory flash speed CFS at different superheats and pressure of flash chamber. It can be seen that there is a minimum NEF with the increasing of circulatory flash speed. Flow rate Q is the main factor that affects circulatory flash speed. Too small flow rate leads to little perturbation, and thus suppresses flash evaporation. Too large flow rate corresponds to short residence time, so there is no enough time for working fluids to evaporate sufficiently. Result suggests that an appropriate circulatory

Fig. 7. CFS versus superheat degree under different pressure of flash chamber.

Y. Wang et al. / Desalination 392 (2016) 74–84

81

Fig. 8. CFS versus superheat degree under different flow rates.

flash speed can prefect flash evaporation. And NEF decreases with the increase of superheat degree and equilibrium, that is to say, flash evaporation is enhanced. In commercial MSF desalination plant, the design of flash chamber occupies an important position. The experimental results show that a minimum value of NEF exists as the circulatory flash speed increases. Although circulatory flash speed CFS increases, the residence time τ decreases. There is no enough time for flash evaporation. The variation of circulatory flash speed with superheat degree, flow rate, water level and equilibrium pressure is obtained. According to this change law, we can design the flash chamber by properly selecting the residence time to get taget NEF·Furthermore, present research indicates that circulatory flash speed decreased to a lowest point, then increased monotonously with the increasing superheat and finally became flat with further increment of superheat degree. It suggests that when superheat degree is high enough, further raising superheat degree has no significant impact on circulatory flash speed. An appropriate superheat degree can make a MSF system more efficient and economical. Comparative research on flash speed between static and circulatory flash evaporation is carried out. Fig. 14(a) shows that the flash speed in circulatory flash is bigger than that in static flash at the same duration time. The explanation is that the circulatory flash evaporation has a

horizontal velocity which results in a turbulence of water film and increases the flash speed. In present range of residence time, the flash speed in circulatory flash evaporation decreases when residence time increases. The longer residence time corresponds to smaller flow rate. So the difference of flash speed with residence time is the same with the changing law under different flow rate. In the previous study [19] of our team, NEF comparison of 5% NaCl solution static and circulatory flash was conducted. And flash speed comparison was plotted in Fig. 14(b) which shows a consistent conclusion with our present study. 3.3. Volumetric heat transfer coefficient (hc) Due to circulatory flash evaporation takes place in the whole water film, a volumetric heat transfer coefficient introduced by J. Yan [15] is adopted to evaluate the boiling heat transfer strength in circulatory flash evaporation. Volumetric heat transfer coefficient, shown in Eq. (12), represents heat flux carried away by flash steam from water film under certain superheat degree.

hc ¼

mfs r ΔT in BLH

Fig. 9. CFS versus superheat degree under different initial heights of water film.

ð12Þ

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Fig. 10. Re versus initial water film height under various flow rates and inlet water temperature.

where mfs is the average mass flow rate of flash steam. On the basis of mass and heat conservation (Eq. (13) and Eq. (14)), mfs can be calculated as Eq. (15). Substituting Eqs. (6), (7) and (15) in Eq. (12), hc can be written as Eq. (16). According to Eq. (11), Eq. (17) can be deduced. Because cpΔTinNEF is much less than latent heat r, Eq. (17) can be simplified as Eq. (18). It suggests that there is a linear positive correlation between hc and circulatory flash speed. And Fig. 15 illustrates that the experimental and calculated results of the relationship between hc and circulatory flash speed. Result suggests that experimental hc grows almost linearly as the circulatory flash speed increases which is the same with theoretical results. But experimental value of hc is greater than that calculated by Eq. (18). It can be explained as follows. Flash evaporation is a type of violent boiling phenomenon. This process produces a large number of bubbles in a short time. Part of fluids is taken up to the condenser by the flash steam. And the mass of the condensed liquid is measured as the mass of flash steam. The theoretical mass of flash steam is lighter than that measured in experiments. So the hc calculated by Eq. (18) is smaller than experimental one. ρin Q in ¼ ρout Q out þ mfs

ð13Þ

Fig. 12. NEF versus circulatory flash speed under different superheat degree.

ρin Q in hin ¼ ρout Q out hout þ mfs hfs

ð14Þ

mfs ¼

ρin Q in cp ΔT in ð1−NEF Þ r−cp ΔT in NEF

ð15Þ

hc ¼

ρin cp ð1−NEF Þr
r−cp ΔT in NEF τ

ð16Þ

ρin cp CFS  hc ¼  cp ΔT in NEF 1− r hc ¼ ρin cp CFS

ð17Þ

ð18Þ

4. Conclusion In the present paper, study on circulatory flash speed (CFS) was carried out based on a set of experiments. The experiments were

Fig. 11. CFS versus superheat degree under different water film concentration.

Y. Wang et al. / Desalination 392 (2016) 74–84

83

Fig. 15. Volumetric heat transfer coefficient versus circulatory flash speed. Fig. 13. NEF versus circulatory flash speed under different pressure of flash chamber.

performed under different pressure of flash chamber, flow rates and initial water film height. 15% aqueous NaCl solution was used as working fluids. The changing law of NEF with various experimental conditions was presented. Result suggests that rising of equilibrium pressure can decrease the NEF. Due to flow rate and initial water level lead to two different changes which have inconsistent effect on NEF, influence of flow rate and initial water film on NEF was not clarified clearly. And thus flash speed was introduced to circulatory flash evaporation. The physical significance of circulatory flash speed is the superheated energy consumed during unit time in flash chamber. Eliminating the influence of residence time, circulatory flash speed can represent the true intensity of flash evaporation. Results suggested that rising of flow rate and equilibrium pressure can enhance circulatory evaporation, and a thinner initial water film height yielded a higher circulatory flash speed. With the increasing of circulatory flash speed, NEF had a minimum value. At the same circulatory flash speed, NEF decreased when superheat degree and equilibrium pressure increased. According to the variation of circulatory flash speed with superheat degree, flow rate, water level and equilibrium pressure, we can design the flash chamber by properly selecting the residence time to get taget NEF.

Comparative study on flash speed between static and circulatory flash evaporation was carried out. Due to the initial turbulence in water film, flash speed in circulatory flash was bigger than that in static flash in the same duration time. And circulatory flash speed decreased with increasing residence time. Finally, volumetric heat transfer coefficient was recruited to evaluate the heat transfer intensity of circulatory flash evaporation. Results showed that volumetric heat transfer coefficient linearly increased with circulatory flash speed. And experimental results of volumetric heat transfer coefficient showed a greater value because of the liquid droplets entrainment.

Conflict of interest statement We declare that we have no financial or personal relationships with other people or organizations that can inappropriately influence our work, there is no conflict of interest in the manuscript entitled “experimental study on circulatory flash speed of aqueous NaCl solution circulatory flash evaporation”.

Fig. 14. Comparison of circulatory flash speed between static and circulatory flash.

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