CFD Modelling of dissolved salt / water flow mixture nucleate pool boiling phase change through vertical bundle of holes

International Journal of Computation and Applied Sciences IJOCAAS, Volume 3, Issue 3, December 2017, ISSN: 2399-4509

278

CFD Modelling of dissolved salt / water flow mixture nucleate pool boiling phase change

through vertical bundle of holes

Mustafa Ahmed Abdulhussain

Abstract-

Index Terms- dissolved salt, two phase nucleate boiling, steam iron

I- Introduction

Wateater is the nerve of life and all its related treatments are necessary either for human, animal, or plant life [1]. In addition, it is now related to industry, energy transfer and storage [2]. Water is used today in solar heaters to store heat absorbed by solar radiation [3]-[4]. It is also used as a saline solution in solar ponds to store the sun's heat for long periods and for many uses [5]-[6].

Water dissolved salts plays a great role in the nucleate boiling phase change phenomena and in the reliability and endurance of relevant devices such as boilers, electric heaters…. etc. This case is complex since it includes the nucleate phase change with heat transfer involved in two different manners, the first is natural convection between flow volumes considering vapor bubble growth, and the second involves conduction with dissolved salt particles.

Thus, the case modeling is clearly intensive in CFD that incorporates multiple parameters such as boiling bulk source influence on vapor bubbles growth rate where the translation of sensible heat transfer from the heaters coil contributes in transient phase change, the dissolved salt minimum and maximum particles size in addition to their flow- conjugated travelling speed.

In addition, the nucleate pool developed pressure, nucleate side density change and the interference criteria between pools with tubes natural/ forced convection are major factors of interest.

Aplenty of research works concerning nucleate pool boiling process have been performed due to the highly importance developing heat transfer correlation-involved factors.

Pezo and Stevanovic [7] predicted a numerical simulation for heat transfer coefficient in nucleate boiling flow with coupling of transient heat conduction with walls; they compared the results with experimental correlations showing a good agreement. They concluded that walls surface roughness have a great influence on the nucleate boiling heat transfer.

Rowinski et al. [8] modeled a non-uniform heat flux distribution applied to a vertical tube flow in supercritical conditions (nuclear reactor) numerically, the trending results approaches good agreement with Ornatsky et al. [9] and Bishop et al. [10] results. The researchers found that tube wall temperature depends higher on the Reynolds number than the operating critical pressure.

Zhang et al [11] have investigated two phase boiling in vertical bundle of tubes at approximately atmospheric conditions experimentally. They developed new correlations for heat transfer coefficient for laminar and Reynolds number for transition regions flow. An important conclusion admitted that forced convection heat transfer instead of nucleate is dominant for boiling heat transfer phenomena.

Water dissolved salts forms a great fouling resistance to heat transfer cases, reduce operating pressure and partially blocks flow channels as well as increasing metal corrosion spreading [12]. In boiling operation, evaporation of liquid will sediment and accumulate the dissolved particles on the heating source body, as with time passing gradual slowdown rate of evaporation and loss of pressure as they have very poor conductivity (below 2W/m² K).

Helalizadeh et al. [12] investigated the salt crystals fouling on heat transfer surfaces during convection heat transfer with sub-cooled boiling conditions focusing on solution salts composition; they predicted the fouling resistance model that showed a good agreement with experiments.

Pool boiling heat transfer coefficient with electrolyte solution measurement was implemented by Ahmadi et al. [13]

focusing on changes in density on nucleation side, bubbles departure diameter with their generation rate. With lower subjected heat flux, the heat transfer coefficient is lower than

W

Mustafa Ahmed Abdul-hussain is now a Lecturer in the Mechanical Engineering Department, University of Technology, Baghdad-Iraq.

Email: [email protected]

279 the pure water measured values and the bubbles departure diameters increased, while the heat transfer coefficient increased with high heat flux values. They introduced a correlation for calculating the boiling heat transfer coefficient with only liquid fraction mass transfer assumption.

The numerical simulation of nucleate boiling transient phase change in the ANSYS CFX code is modelled estimating constant heating source flux. The investigation of pure and seawater salts dissolved two-phase volume mixture with (100°C) saturation temperature in steam iron half-filled vessel carried out. Applying Schiller Naumann drag coefficient for modeling momentum transfer between the fluid phases and the thermal phase change model for the mass transfer resulted from the boiling phenomena. Adopting two-resistance model for the two-phase heat transfer exchange Dissolved salts is considered as a particle transport solid having the same flow travel speed. Steam iron vessel is half - filled of water covering the heating element considering opening inlet boundary condition, the mixture flows through 8 vertical holes to the free stream. The simulation of the phase change phenomena with transient mass transfer occurrence for both pure two-phase flow and dissolved salts/flow mixture considering steam iron contributed walls thermal conduction heat loss.

II-

Figure (1) illustrates the steam iron including the water vessel that is half-filled in addition to the heating element sketched with SOLIDWORKS 2016 considering the vessel enclosure is pressurized (2bar). The heating element has a U- shape circular coil of (1.5 cm) diameter. The vessel dimensions are (15*15*10), with the eight flow holes diameter and length of (1 & 5) respectively.

Fig.1 The case geometry

The governing solved equations for both phases are the continuity, momentum and energy in addition to the heater walls energy equation

The continuity equation

The momentum equation

The energy equation

The heated wall energy equation

The pool flow is considered laminar (natural circulation) with turbulent flow through tubes; the well-known K-Ɛ scalable turbulence model is utilized

   

   







 



 

 

 

 



2

. 1

.

. .

C P k C t

P t A

k

k t

k k

t







 



 

 



 







 









 



 

 



 







 



U U A

The thermal phase change model considers interface heat transfer between phase sides (the two-resistance model). The sensible heat flux from the heating source for the liquid phase (a) and vapor phase (b) assuming no mass transfer will be:

𝑞_𝑎= ℎ_𝑎(𝑇𝑖 − 𝑇𝑎) 𝑞_𝑏= ℎ_𝑏(𝑇𝑖 − 𝑇𝑏) Where the subscripts (a & b) represents the liquid and

vapor phases.

(Ti)is the volumes interfacing temperature, which is equal to the saturation temperature-neglecting phase’s volume surface tension. In our case, the mass transfer is a major involved criterion, the interphase mass transfer for both phases considering sensible heat balance (𝑞𝑎 = 𝑞𝑏)

𝑄𝑎 = 𝑞_𝑎+ 𝑚_𝑎𝑏. 𝐻_𝑎𝑠 𝑄_𝑏= 𝑞_𝑏− 𝑚_𝑎𝑏. 𝐻_𝑏𝑠 Where (mab) is the mass flux from phase (b) and (Has, Hbs)

represents the enthalpy difference for each phase.

The phase transition requires defining continuous and dispersed phases. In our case, the liquid is the continuous

280 phase with vapor dispersion (bubbled flow), one of the most factors arises among interaction of the two phases is the fluid volume drag function, the Schiller-Naumann correlation for drag coefficient have been adopted in the simulation

𝐶𝑑 = 24(1 + 0.15𝑅𝑒^0.687) For 0 ≤Re≤1000 𝐶𝑑 = 0.44 For Re>1000 The predicted phase heat transfer closely related to the

phase Nusselt number. The two Resistance model implemented in the solution using the Ranz-Marshall correlation equation (12) for liquid phase heat transfer that is the most suitable for spherical bubbles formation [8].

𝑁𝑢_𝑎= 2 + 0.6𝑅𝑒^0.5 𝑃𝑟^0.33 For continuous liquid boiling process with constant

saturation conditions, the zero-resistance vapor heat transfer model is the most appropriate one [8].

Figure (2) denotes the hex-dominant un-structured mesh capable to handle unlimited size of grids generated with statistics of more than (250000) nodes and (1150000) elements using the ANSYS ICEM CFD meshing code

Fig.2 Generated mesh

The boundary conditions applied to the case solution are as follows:

1- Assuming the flow mixture initial saturation pressure and temperature are (2 Bar) and (100°C). The heating element has a fixed temperature of (100°C). To shorten the time of simulation, the mixture is assumed of equally (50%) initial volume fraction

2- An inlet opening boundary condition was adopted in order to preserve the mixture saturation properties. The mixture outlet is exposed to free atmospheric conditions neglecting still air heat transfer effect.

3- The steam iron body initial temperature is set to (25°C) with stainless steel material, considering convectional boiling heat transfer coefficient with flow mixture to walls

(3000W/m²K) (boiling heat transfer range from 3000-100000 [8]).

4- Salt dissolved particles is considered as a conjugated flow injected travelling solid having mean diameter range of (0.001-0.002m) treated as a homogeneous solid with sea salts thermal properties.

Selecting the high resolution advection scheme with second order transient approach, working to desired root mean square residual (RMS) to [1e-4], the solution time periods have been taken 10 sec,30 sec and 60 sec(1 minute) with time step 0.5 second.

III-

Figures (3-13) represents the pure water liquid/vapor two phase characteristics transient behavior with time, figures (3)

& (4) clearly illustrates the thermal balance for each phase at time step (t=10 sec); moreover, balanced volume fraction is observed through tubes bundle.

Fig.3 liquid volume fraction gradient change at t=10 sec

The vapor-phase temperature contours near wall is influenced by the natural convection phenomena as noticed in figure (5) remarking the gradual descending temperatures from upper to lower control volume surfaces.

Fig.4 vapor volume fraction gradient change at t=10 sec

281

Fig.5 vapor temperature contour at t=10 sec

Figure (6) shows the steam iron temperature distribution denotes the maximum on the vessel upper surface side due to concentrated vapor fraction at that region. Since the two-phase flow, behavior is density- changed.

Fig.6 steam iron temperature distribution

Density variation of the volume fractions is illustrated in figure (7) noticing less density (vaporized phase) close to the upper control volume.

Fig.7 two phase mixture density change at t=10 sec

As the time undergoes 30 & 60 seconds, more changes in the mixture volume fractions, temperature distribution and density change becomes clearer in figures (8-13).

Fig.8 vapor volume fraction gradient change at t=30 sec

Fig.9 vapor temperature contour at t=30 sec

Fig.10 two phase mixture density change at t=30 sec

282

Fig.11 vapor volume fraction gradient at t=60 sec

Fig.12 vapor temperature contours at t=60 sec

Fig.13 two phase mixture density change at t=60 sec

B- Dissolved salt/two phase mixture

The injected salt with time step greatly affects the vapor fraction consistence resulting gradually in increasing bubbles formation resistance with time since the initial evaporation will decrease the liquid volume and hence, the salt fraction

will furthermore slowdown the evaporation process. Figures (14), (15) and (16) review the changes in vapor fraction.

Fig.14 vapor fraction for salt/fluid mixture at t=10 sec

Fig.15 vapor fraction for salt/fluid mixture at t=30 sec

Fig.16 vapor fraction for salt/fluid mixture at t=60 sec

283 IV. CONCLUSIONS

In this article, the mathematical modeling of the nucleate pool boiling using the two-resistance phase change model was used by applying the Ranz-Marshall correlation constitutes an appropriate solution technique for multi-phase change simulation cases. The study results concludes that it is obvious that the salts contribute negatively by slowing down the thermal phase change process, decreasing temperature gradient.

NOMENCLATURE A kinetic energy

Cd Drag coefficient

F Interfacial drag force (N/m³) g gravity acceleration (m/s²) K Thermal conductivity (W/m².K) Cp Specific heat

H Convection heat transfer coefficient Nu Nusselt number

P Pressure (Pa) Pr Prandtle number

qb volumetric heat source for bubble generation on the heater’s surface (W/m³)

qh volumetric heat rate (W/ m³) Re Reynolds number

T Temperature (°C) u Velocity (m/sec)

Greek symbols

ρ Density (Kg/m³)

Γ Phase transition rate, (kg/ m³ s)

α Void fraction

μ Dynamic viscosity, (Pa·s)

σ Surface tension, (N/m)

Subscripts

k Phase index

b Bubble

c Condensation

e Evaporation

p Particle

REFERENCES

[1] K.I. Abaas, M.T. Chaichan, "Experimental Study of using Solar Energy Storage Wall for Heating Iraqi Houses Purposes," Wassit Journal for Science & Medicine, vol. 2, No. 2, pp. 212-221, 2009.

[2] H.K. Kazem, H.S. Aljibori, F.N. Hasoon and M.T. Chaichan,

"Design and Testing of Solar Water Heaters with its Calculation of Energy," Int. J. of Mechanical Computational and Manufacturing Research, vol. 1, No. 2, pp. 62-66, 2012.

[3] M.T. Chaichan, H.A. Kazem, "Using Aluminum Powder with PCM (Paraffin Wax) to Enhance Single Slope Solar Water Distillator Productivity in Baghdad-Iraq Winter Weathers," International

Journal of Renewable Energy Research, vol. 1, No. 5, pp. 151-159, 2015.

[4] M.T. Chaichan, H.A. Kazem, "Water Solar Distiller Productivity Enhancement using Concentrating Solar Water Heater and Phase Change Material (PCM)," Case Studies in Thermal Engineering, Elsevier, vol. 5, pp. 151-159, 2015.

[5] M.T. Chaichan, K.I. Abaas, "Productivity Amelioration of Solar Water Distillator Linked with Salt Gradient Pond," Tikrit Journal of Engineering Sciences, vol. 19, No. 4, pp. 24-34, 2012.

[6] M.L. Pezo and V.D. Stevanovic, "Numerical Prediction of Nucleate Pool Boiling Heat Transfer Coefficient under High Heat Fluxes", Thermal Science Journal, vol. 20, pp. 113-123, 2016.

[7] M.K. Rowinski, J. Zhao, T.J. White, Y.C. Soh, “Numerical Investigation of Supercritical Water Flow in a Vertical Pipe under Axially Non-Uniform Heat Flux,” Progress in Nuclear Energy, vol.

97, pp. 11-25, 2017.

[8] A.P. Ornatsky, L.P. Glushchenko, and E.T. Siomin, “The Research of Temperature Conditions of Small Diameter Parallel Tubes Cooled by Water under Supercritical Pressures”, in Proceedings of the Fourth International Heat Transfer. 1970. Paris-Versailles, France.

[9] A.A. Bishop, R.O. Sandberg, L.S. Tong, "Forced Convection Heat Transfer to Water at Near-critical Temperatures and Supercritical Pressure," P.I. Report WCAP-2056, Editor: Westinghouse Electric Corp., Pittsburgh, USA. 1964.

[10] K. Zhang, Y.D. Hou, W.X. Tian, Y.Q. Fan, G.H. Su, and S.Z. Qiu

"Experimental Investigations on Single-Phase Convection and Steam-Water Two-Phase Flow Boiling in a Vertical Rod Bundle,"

Experimental Thermal and Fluid Science, vol. 80, pp. 147–154, 2017.

[11] A. Helalizadeh, H. Müller-Steinhagen, M. Jamial Ahmadi,

"Mathematical Modeling of Mixed Salt Precipitation During Convective Heat Transfer and Sub-Cooled Flow Boiling, Chemical Engineering Science, vol. 60, pp. 5078 – 5088, 2005.

[12] M.J. Ahmadi, A. Helalizadeh, H. Muller-Steinhagen, "Pool Boiling Heat Transfer to Electrolyte Solutions", International Journal of Heat and Mass Transfer, vol. 47, pp. 729–742, 2004.

[13] ANSYS CFX 17.0 Documentation, 2016.