Numerical simulation of processes in combined shock-foam-type air-handling unit with a block of thermoelectric modules Текст научной статьи по специальности «Физика»

Section 11. Technical sciences

Рис. 3. Локализация импульсов АЭ по длине рабочей части дефектного образца с усилением шва между преобразователями

Рис. 4. Локализация импульсов АЭ по длине рабочей части дефектного образца без усиления шва между преобразователями

Список литературы:

1. Хромченко Ф. А. Типичные повреждения и ремонт сварных соединений паропроводов из хромомолибденованадиевых сталей/Ф. А. Хромченко, В. А. Лапа, В. Г. Гриненко//Теплоэнергетика. - 1993. - № 11. -С. 18-21.

2. Хромченко Ф. А. Ресурс сварных соединений паропроводов/Ф. А. Хромченко. - М.: Машиностроение, 2002. - 352 с.

3. Гофман Ю. М. Оценка работоспособности металла энергооборудования ТЭС/Ю. М. Гофман. - М.: Энер-гоатомиздат. - 1990. -245 с.

Chaban Inna Victorivna Kyiv National University of Construction and Architecture postgraduate student Department of engineering systems and ecology

E-mail: shadyra.inna@gmail.com

Numerical simulation of processes in combined shock-foam-type air-handling unit with a block of thermoelectric modules

Abstract: Article present an analysis performed with the FLUENT code, aimed at improving the understanding of hydrodynamics, heat and mass transfer mechanisms that occur in shock-foam-type air-handling unit.

Keywords: numerical simulation, combined shock-foam-type air-handling unit, mass transfer, heat transfer, Euler-Euler model.

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Numerical simulation of processes in combined shock-foam-type air-handling unit with a block of thermoelectric modules

Introduction

The problem of microclimate in museum premises is a complex task. On the one hand it is creation of such conditions that would ensure long term of preservation of museum specimen, on the other hand creating a comfort conditions for visitors and employees of the museum. At the present time, the optimal parameters of the microclimate in the premises of the museum are usually created using central or self-contained air conditioning systems with air-handling and air-conditioning equipment. Besides of the exhibition halls, in museums are present workrooms, laboratories, research premises, museum depositories and other. There is own, specific requirements for microclimate parameters in each of these areas, that demand instrumentation of self-contained air conditioning units, to perform the air-handling process under each of the specific areas. We have developed and patented a combined shock-foam-type air-handling unit with a block ofthermoelectric modules for such areas [1]. Combined shock-foam-type air-handling unit provides an optimal microclimate parameters over a wide range and ensures efficient air purification from various types of pollution, however is compact, lightweight, energy efficient and makes possible a temperature and cooling capacity variable control. The handling belong to surface trickling heat exchangers, which combine a contact and surface heat-exchangers in one design. An air-handling unit works on the principle of the creation dynamic foam layer that flow over the surface of heat exchangers. Strong turbulence of gas-liquid system provides a heavy increase of interacting phases of contact surface which lead to intensification ofheat-mass-exchange between air and water and for efficiency upgrading of air purification from pollution. The processes in combined shock-foam-type air-handling unit present a hard problem for numerical modeling because of the complex phenomena of heat and mass transfer in turbulent foam layer.

The processes of heat and mass transfer in the foam layer has been studied by a number of researchers. Generally mathematical modeling was reduced functional relationships between the parameters defining the processes and pacing factors using similarity theory and experimental researches.

Today, the basic hydrodynamic equations complete with boundary condition could be solved using numerical methods. Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat transfer, mass transfer, chemical reactions, and related phenomena by solving the mathematical equations that govern these processes using numerical algorithms. The results of

CFD analysis are relevant engineering data used in conceptual studies of new designs, detailed product development, troubleshooting, and redesign and therefore CFD is gaining importance in general process applications. CFD approaches use numerical techniques to solve the Navier-Stokes equations for given flow geometry and boundary conditions there by implementing models for flow aspects like turbulence or heat and mass transfer as relevant for the specific modeling task [3, 80-90].

The fluid dynamics of these systems has been widely studied with CFD, by resorting to different multi-phase models, notably the multi-fluid approach. This and other approaches require however an a priori assumption on the mean bubble size to be used and in the simulation, and are strongly limited by the assumption that this value is constant throughout the computational domain [3, 5-80].

Generally gas-liquid systems of bubbling columns has been simulated. Air velocity in the cross section of shock-foam-type air-handling unit is in 5-10 times higher than in bubbling column. Therefore is necessary to check the consistency of the models and the importance of assumptions and constants for the foam system [8-12].

The goal of this work was to study the air-water behavior and heat transfer between surface of heat exchangers and foam layer inside a shock-foam-type air-handling unit with block of thermoelectric modules.

Computational model

This paper studied a shock-foam-type air-handling unit using CFD. The commercial software FLUENT based on the finite volume method (FVM) was used to simulate air and water flows in Combined shock-foam-type air-handling.

To describe the flow of gas and liquid phases was used, the Euler approach for multiphase flow. The movement of each phase is governed by the relevant continuity equation, conservation of momentum, energy and species.

In the present work, an Eulerian-Eulerian multiphase model is adopted where gas and liquid phases are all treated as continua interpenetrating and interacting with each other, everywhere in the computational domain. The pressure field is assumed to be shared by all the two phases, in proportion to their volume fraction. The motion of each phase is governed by respective mass and momentum conservation equations [8, 50-80].

Continuity equation

The continuity equation describes the mass flux into and out of a control volume. The volume-average continuity equation for k-th phase:

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Section 11. Technical sciences

d n

dt (akPk )+v-(akPkuk )=T,(mPk- mkp) (i)

p=

Де pk,uk,ak represent, respectively, the macroscopic density, velocity, and volume fraction of phase k, while, mpk is mass transfer from the p to k-th phase and mkp is mass transfer from the k to p-th phase, respectively.

Momentum equation

In multiphase formulation, momentum balances similar for continuous and dispersed phases. The momentum balance for k-th phase in most general formulation is:

d =

dt (akPk uk ) + V '(akPk uk uk) = ~akV p + Vrt +

n

+akPkg + X(mpkvpk - mkpukp) + Fk

p=i

where, g, P, Fk are gravity acceleration, hydrodynamic pressure, and the momentum exchange term, while Tk is the viscous stress tensor of k-th phase stress which is given by:

= (oTh)( + Vvl) + ) - 2 ^ "jvvkl, (3)

where, xk and kk are the shear and bulk viscosity of phase k and I is a unit tensor.

Energy Equation.

The energy is calculated as follows:

4- (ОсРЛ ) + v • (ерс ukhk) = ~ak 44+

dt dt /

_ n , (4)

+VrkVOk + Vqk + YJ(Qpk + mpkhpk - mkphkp) p=1

(2)

where, hk is the sensible enthalpy ofphase k; qk — is the heat flux; Qpk is the intensity of heat exchange between phases; hpk — is the interphase enthalpy.

Species transport model

FLUENT can model the mixing and transport of species by solving conservation equations describing convection and diffusion for each component species. In this project, water vapour can be considered specie distributed inside shock-foam-type air-handling unit. Conservation equation takes the following general form: d

-H) + V-(poy.) = -VJ. + s„ (5)

In this equation, Y is mass fraction of each species i, St is the rate of creation by the addition of the dispersed phase. The production of species by chemical reaction was also neglected. The flow of mass diffusion equation used in the energy and species conservation equation is calculated as follows:

J,

Sct у

vy ,

(6)

where, Sa is the turbulent Schmidt number which is given by, being the turbulent viscosity and Dtm turbulent diffusivity.

Therefore, there are two species in the CFD model — water vapour and air. Water vapour should be the first specie and the air should be the second specie. Then FLUENT can calculate mass diffusion in turbulent flows and species transport in the energy equation.

Turbulence model

Additional transport equations for the turbulent kinetic energy k and its dissipation rate £ were considered. Here we considered the realizable per-phase turbulence model. Transport Equations for kinetic energy k and its dissipation rate £:

dt

(та )+v-(akPkUkh)

d

f ( T

V ak Ek + — < PT-

V V CTk ) )

(akGK,k akPk£k ) + Kpk ' pk ' kp Ckp ' kk ) (7)

-K,

,(U p-U k)

E,

—Vap + K

ap°p

d

dt

(akPk sk ) + V-(akPkUk sk ) = V

4p (U ( -17 k ) E ,k Vak

>ak k

f f

V ak E + Et ,k Vsk

'v E ) )

C 2sakpk

kk+4

V, ,pEk

+ С. C3 -±Kp x

Is 3e i kp k

(8)

(Cpk • kp - Ckp •kk)- Kkp (U p-17 k )• Vap +

aPaP

K

Up -Vk)

ab ^k

Vak

where, Uk is the phase-weighted velocity, GK,k is the turbulence kinetic energy generated due to the mean velocity gradient, Kpk the turbulent drag term for multiphase flowtion. C1s ,C 2s and С 3s are constants. ok and aE oe are the turbulent Prandtl numbers for k and £, respectively.

The turbulent viscosity, pt, is computed by combining k and £ as follows:

E ,k =PkC p~

(9)

s

k

2

k

Initial and boundary condition Three dimensional computational geometry of the air-handling unit have been generated by using ANSYS Design modeler. After geometry creation, a uniform mesh has been generated with map structured quadrilateral elements containing height to width ratio of 1.

In order to obtain a well-posed system of equations, reasonable boundary conditions for the computational domain have to be implemented. Inlet boundary condition is a uniform liquid and gas velocity at the inlet, and outlet boundary condition is the pressure

124

Numerical simulation of processes in combined shock-foam-type air-handling unit with a block of thermoelectric modules

boundary condition, which is set as 1.013-10 5 Pa. Wall boundary conditions are no-slip boundary conditions for the liquid phase and free slip boundary conditions for the gas phase. The condition of symmetry of the problem allowed simulation of only half of the

geometry, thus establishing a vertical plane in the axis of the air-handling unit. On that plane, symmetry boundary condition was applied. At initial condition the volume fraction of the gas at inlet and in the free board region is based on the inventory.

Fig. 1. Boundary condition for numerical simulation processes in shock-foam-type air-handling unit

The Phase Coupled SIMPLE method has been chosen for pressure-velocity coupling. The second-order upwind scheme has been used for discretization of momentum, turbulence kinetic energy and turbulence dissipation rate and the first-order upwind scheme has been used for discretization of volume-fraction equations. The time step size of 0.001s has been used. Relaxation factors for pressure, density, momentum, volume fraction have been used according with recommendation: 0.3, 1, 0.2, 0.5, respectively. The simulations have been carried out till the system reached the quasi-steady state.

аЯ| ini

9.W-HE

33V+Ш ' ‘I 9№4Ш

тзг*+ш

T+ti+CE GSTc-fffl

6.4&-HE 59&+Ш S4&+TE + S&+C?

4.45++Щ

Э.4&+С2 2 33? 402 2+4«+02 194++Ш I +4« +03 9Л++01 + Э4++С1 -БЯЭе+Ш

Results and discussion

To make image of processes in shock-foam-type airhandling unit during simulation pressure and velocity fields have been obtained. The pressure loss of air flow in shock-

foam-type air-handling unit is a factor of great importance because it directly influences the costs of manufacturing and operating such equipment. The pressure loss depend on design and hydrodynamics features of air-handling unit. Design factors include the distribution channels configuration, their location relative to the working space of air-handling unit and heat exchangers geometry. Hydrodynamics factors include static water level and air velocity in the cross section of the air-handling unit.

Я ; -on Tjsat+ш T.1A-KD GT&-KS3!

C-.lifc-ЩР

j

2Т1Ч-КЕ 1

1 5ft+ф

1.19« фШ I Зк-П1

Fig. 2. Pressure and velocity fields on vertical axis plane of the air-handling unit.

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Section 11. Technical sciences

Fig. 1 shows the pressure and velocity fields have been obtained from the simulation. The pressure in air-handling unit is reduced with the height ofthe same, being smaller the closer it is to the exhaust air, as expected. The highest pressure drop and air velocity were at the inlet slot-shaped channel. Pressure drop intensifies when passing the heat exchanger and foam layer. The highest air velocity were when air stream pass heat exchanger placed on the middle of air-handling unit. That’s because on the space between heat exchanger fins present small diameter pipes that lead to high pressure drop.

Air humidification, dehumidification with cooling or heating processes could be realized in shock-foam-type air-handling unit. Fig. 1 shows the temperature field on vertical axis planes of the air-handling unit and on surface of heat exchanger fins. We detected that processes which take place in the working space of air-handling unit were the most intensive when the static water level were 20 mm and air velocity in the cross section of the air-handling unit were 3 m/s.

Fig. 3. Temperature field on vertical axis planes of the air-handling unit and on surface of heat exchanger fins at the static water level h = 10 mm, and the air velocity in the cross section 3 m/s

Numerical simulation allows to follow the processes in the air-handling unit. Fluid turbulization around the fins of the heat exchanger lead to partial destruction of laminar boundary layer on the heat exchanger surface. As

1

result surface heat transfer coefficient of heat exchanger placed in from layer is in 100-600 times higher than in air stream.

I I t

' F '

I. 1 ' '

-1 ' l

■ Vi

Fig. 4. Air velocity vectors in the cross section of the air-handling unit

While heat transfer efficiency of the heat exchanger placed in from layer is high, design of heat exchanger has several disadvantages. The heat exchanger fins, which are

placed in underpart of air-handling unit create constant cross-section that provide uneven flow around the heat exchanger surface as shown on Fig. 4. As result reduced

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Numerical simulation of processes in combined shock-foam-type air-handling unit with a block of thermoelectric modules

heat transfer efficiency from foam layer to the heat exchanger surface.

Design features that reduce the shock-foam-type air-handling unit efficiency, were detected during the numerical simulation. The heat transfer from air stream to the heat exchanger placed at the top of air-

handling unit increase when the air velocity in the cross section of the air-handling unit adopt a value which exceed 4.5 m/s. Air relative humidity after the foam layer ranging 85-98%. It is degrees when air stream pass the heat exchanger placed at the top of air-handling unit.

Fig. 5. Relative humidity at the outlet of of the air-handling unit

When air velocity increase to 4.5 m/s air relative humidity began increase too. That is because with air velocity increasing water drops that become entrained

from foam layer by the air stream, as shown on Fig. 6. Water drops that getting the heat exchanger surface evaporate and intensify the heat transfer processes.

Volume

fraction

(water)

1 Шё+Ш 9Ше-П1

вше-ш

ТДК-Ш

5ДК-Ш

5Ше-П1

Шк-Ш

ЭШе-П1

2Ш?-П1

Шк-Ш

□Пк-МЛ

Fig. 6. The volume fraction of water in work space of the air-handling unit at the static water level h = 20 mm, and the air velocity in the cross section 4.5 m/s.

This phenomena is positive in sense of air-handling unit efficiency upgrading only on that case when the process of air humidification with air cooling or heating are realized and high level of relative humidity is acceptable.When the process ofair dehumidification with or without air cooling are realized air humidification is undesirable.

Model validation

Results obtained from numerical simulation were compared with experimental data. Fig. 7 presents a comparison the Number of transfer units as a function of air Reynolds number and static water level, evaluated from experimental results through CFD simulations of air-handling unit.

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Section 11. Technical sciences

Fig. 7. Number of transfer units as a function of air Reynolds number and static water level, evaluated from experimental results through CFD simulations of air-handling unit

Fig. 8 presents a comparison of the Nusselt number which characterize heat transfer coefficient evaluated by

CFD simulations and by experiment.

Fig. 8. Nusselt number as a function of air Reynolds number evaluated from experimental results through CFD simulations for heat exchanger which is placed in underpart of air-handling unit

The correlation for the heat transfer coefficient obtained through CFD was close to the experimental correlation, especially for higher air flow rates. The minimum deviation for Number of transfer units was 2% and the maximum deviation was 8%, the minimum deviation for Nusselt number was 4% and the maximum deviation was 12%, which is acceptable for a two-phase turbulent flow.

Conclusions

1. Results obtained from numerical simulation helped to detect regions with the highest pressure drop of air flow, lower-range value of heat transfer coefficient and design features that reduce the shock-foam-type airhandling unit efficiency.

2. Results obtained from numerical simulation were compared both qualitatively and quantitatively

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Numerical simulation of processes in combined shock-foam-type air-handling unit with a block of thermoelectric modules

with experimental data and show good convergence.

3. To reduce pressure drop of air flow and to increase heat transfer efficiency in shock-foam-type air-handling unit it need to improve aerodynamic design ofshock-foam-type air-handling unit and construction ofheat exchangers.

4. The extended rating of processes and air stream conditions in shock-foam-type air handling

unite we must prevent drop entrainment and their getting the heat exchanger surface. Increase in vertical dimension between the foam layer and heat exchanger placed at top of air-handling unit or drift eliminator installation can provide a more effective cooling and dehumidifying of air before reaching the heat exchanger.

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