CFD Analysis of Temperature-stratified Water Thermal Energy Storage Tank- Modeling of Punched Metal Attached to Top Surface of Diffuser -

2.2 Modeling of punched metalIn the previous studies, punched metal has been modeled by the 1-dimensional pressure drop model, which has been applied to cell face in the process of modeling, so that pressure drops can occur in only vertical direction. In the 1-dimensional pressure drop model setting, diagonal flow was discharged from inlet diffuser through punched metal. However vertical flow behavior was observed in flow visualization in the previous experiment of the water tank as shown in Fig.3, which showed flow behavior of the previous model disagrees with that of visualization. Trying to solve this problem, the authors proposed the 3-dimensional pressure drop model. It can be applied to cell volume in the process of modeling, so that pressure drops can occur in horizontal direction in addition to vertical direction. The pressure drops of these models are defined in Eq. (1)2)3).

In the 1-dimensional pressure drop model, we set pressure drop only to the vertical direction and in the 3-dimensional pressure drop model, we set pressure drop in both vertical and horizontal directions. In this study, the coefficients (α and C) in Table.2 were applied, which have been obtained in the previous research. 2.3 CFD analysis case/conditionThree cases of isothermal steady state analysis were conducted: Detailed low-Re, 1-dimensional pressure drop model, and 3-dimensional pressure drop model. In isothermal steady state analysis, only one case (flow rate 30 L/min) is studied to discuss accuracy of modeling method for punched metal. The result of flow behavior obtained from Detailed low-Re is here regarded as true value, and compare the 1-dimentional pressure drop model to the 3-dimensional pressure drop model based on Detailed low-Re. It seemed rational that horizontal pressure drop is infinitely large. Given the stability of calculation, however, we set horizontal pressure drop

1. Introduction The temperature-stratified water Thermal Energy Storage (TES) tank is a part of the TES air conditioning system which has been installed in many buildings because it can contribute to saving energy and electric-load leveling. In the temperature-stratified water TES tank, high-temperature water is stored in the upper part, and low-temperature water is stored in the lower part almost without mixing by utilizing water density difference, and its performance is evaluated on the basis of temperature gradient in temperature-stratified layers. The configuration of diffusers is one of the important factors to maintain high performance of the tank, because good design of diffusers can reduce mixing of water in the tank, which has been studied by many researchers. The purpose of this study is to discuss method of punched metal attached to diffusers of vertical inflow type. Its role is to obtain uniform velocity distribution. In the previous studies, punched metal has been modeled by the 1-dimensional pressure drop model. However it was found that flow behavior of this model has not agreed with that of the flow visualization by experiments1). This study compares this model with the 3-dimensional pressure drop model, which is proposed as a new method showing a good agreement with experimental results. First, we conducted isothermal steady state analysis to discuss modeling of punched metal, and after that conducted non-isothermal unsteady state analysis varying inflow temperature difference to compare heat storage performance between two models.2. Isothermal steady state analysis2.1 CFD analysis domainThe temperature-stratified water TES tank calculated in steady state analysis is shown in Fig.1. A diffuser of vertical inflow type is located at the horizontal center of the tank. Punched metal is shown in Fig.2. A quarter of the tank was calculated by using symmetry boundary condition to shorten time required for calculation.

Ayako Higuchi 1, Tomohiro Kobayashi 2, Takeshi Iwata 3, Kazunobu Sagara 1,

Toshio Yamanaka 1, Hisashi Kotani 1, Yoshihisa Momoi 1,

Osamu Koga 4, Kyohei Ichitani 4, and Mitsuru Nishiyama 5

1Department of Architectural Engineering, Graduate School of Engineering, Osaka University, Suita, Japan2Department of Urban and Building Engineering, Graduate School of Engineering, Osaka City University, Osaka, Japan

3Department of Architecture, Graduate School of Engineering, Mie University, Tsu, Japan4The Kansai Electric Power Co., Inc., Osaka, Japan

5Taikisha Ltd., Osaka, Japan

p v C v nii

i= − +( )µα

ρ 212

ΔΔ i x y z= , ,

ABSTRACT

(1)

as 1000 times larger than vertical pressure drop in the 3-dimensional pressure drop model. Besides, it may be assumed that setting of 1-dimensional pressure drop model means horizontal pressure drop is zero because we found horizontal component vector is much larger in the case of 1-dimensional pressure drop model. In order to confirm that, we compare the 1-dimensional pressure drop model to the 3-dimensional pressure drop model with horizontal component of pressure drop zero (Horizontal Zero Setting). All analysis cases are listed in Table 1, conditions are in Table 2, and setting of coefficient values is in Table 3.2.4 CFD analysis result and discussionFig.5 and 6 show velocity distribution around inlet diffuser. Vertical flow can be seen in the 3-dimensional

280m

m

350mm

80m

m

Di�user Tank wall

Inlet

Water surface

OutletSymmetry

1/4 tank ZY section

Z

Y

1/4 tank XY plan

350mm

350m

m

125mm

50mm

Di�user

Symmetry

Tank wall

X

Y

700mm700mm

280m

m

ZXY

1/4 tank

Di�user

Whole tank

Punched metal

Box di�userInternal：100mm×250mm×50mmOuter ：105mm×255mm×55mm

Vertical pipeInternal：50mm × 50mmOuter ：55mm × 55mm

Diffuser

staggered

Analysis method Detailed analysis Pressure drop model

Case Name Detailed Low-Re

3-D Pressure Drop

Model1-D

Pressure Drop Model

(3-D model)Horizontal Zero

Setting Turbulence model Low-Re k-ε RNG k-ε RNG k-ε RNG k-εNumber of mesh 2,047,597 723,467 723,467 723,467

Pressure drop model -

3-dimensional pressure drop

model1-dimensional pressure drop

model3-dimensional pressure drop

model

Table 1 CFD analysis caseFig. 1 Analysis domain

Fig. 2 Diffuser and Punched metal

Fig. 3 Flow visualization by the previous study1)

Punched metal

Direction

Face Volume

De�ne volume as �uid zone Set pressure drop per unit length

Vertical and horizontal

Interior boundary condition

Vertical

3-D pressure drop modelModel 1-D pressure drop model

Subject

Di�user section

Setting

3-D pressure drop model1-D pressure drop model

of pressure drop

Fig. 4 1-D and 3-D Pressure Drop Model

CFD Code FLUENT14 Discretization Scheme QUICK

Algorithm Steady state(SIMPLE)

Boundary Condition

Inlet Define flow rateOutlet Define flow rate

Tank Wall Wall:Generalized log lowWater Symmetry:free slip

Table 2 CFD analysis condition

Table 3 Pressure drop model condition

Face Permeability α[m2] 3.90E-09(ViscousResistance) (1/α) [1/m2] 2.56E+08

Thickness Δｎ [m] 1.00E-03Inertial Resistance C [1/m] 2.60E+04

1-Dimensional Pressure Drop Model

Viscous Resistance

(1/αx) [1/m2] 2.56E+11(1/αy) [1/m2] 2.56E+11(1/αz) [1/m2] 2.56E+08

Inertial Resistance

Cx [1/m] 2.60E+07Cy [1/m] 2.60E+07Cz [1/m] 2.60E+04

3-Dimensional Pressure Drop Model

Viscous Resistance

(1/αx) [1/m2] 0(1/αy) [1/m2] 0(1/αz) [1/m2] 2.56E+08

Inertial Resistance

Cx [1/m] 0Cy [1/m] 0Cz [1/m] 2.60E+04

(3-D Model) Horizontal Zero Setting

pressure model as well as Detailed low-Re. This indicates that flow behavior of the 3-dimensional pressure drop model shows good agreement with experiment. We can also see almost similar velocity distribution between 1-pressure drop model and the 3-dimensional pressure drop model with horizontal

component of pressure drop zero (Horizontal Zero Setting), so that it is confirmed that no pressure drop term is added in horizontal direction on the 1-dimensional model. 3. Non-isothermal transient analysis 3.1 CFD analysis outlineAfter discussing flow behavior in isothermal steady state analysis, we confirm heat storage performance in non-isothermal unsteady state analysis changing the inflow temperature. Here vertical temperature distribution was used to compare two models in contrast with flow visualization from the previous study. In the CFD analysis, all configurations, such as tank,

diffuser, punched metal, and all conditions such as inlet temperature, flow rate and distance between water surface and top surface of diffuser (Installation depth of diffuser) are the same as those of experiment. 3.2 CFD analysis domainThe temperature-stratified water TES tank calculated by unsteady state analysis is shown in Fig.7. The diffuser of vertical inflow type is located at the horizontal center of the tank. The specification of punched metal is: pore diameter (D) = 1mm, interval( P) = 2mm, thickness (Δn) = 1mm, and 60 degree-staggered arrangement4). A quarter of the tank was calculated by using symmetry boundary condition to shorten time required of

Horizontal Zero Setting (3-D) 1-D Pressure Drop Model3-D Pressure Drop Model Detailed Low-ReFig. 5 Comparison of flow velocity distribution (ZX section)

Fig. 6 Comparison of flow velocity distribution (YZ section)Horizontal Zero Setting (3-D )1-D Pressure Drop Model3-D Pressure Drop Model Detailed Low-Re

Table 4 CFD analysis case Table 5 CFD analysis conditionCFD code FLUENT14

Turbulence model RNG k-ε model

Number of meshcase 1 6 613392case 7 12 661504case 13 18 703602

Discretization scheme Second order upwindAlgorithm Transient state(SIMPLE)

Time step sizecase 1 2 7 8 13 14 1s

case 3 4 9 10 15 16 0.5scase 5 6 11 12 17 18 0.33s

Density Polynomial quartic function

Boundary

condition

InletOutlet

Tank wall Wall:Generalized log lowWater surface Symmetry: free slip

Punched-metal Pressure drop model

Difine flow rateDifine flow rate

Experiment CFD case Installation depth[mm]

Initial temperature[ ]

Inlet temperature[ ] Pressure drop model

experiment1 case1 29.3

50

17.9 38.81-D model

case2 3-D model

experiment2 case3 59.4 22 32.21-D model

case4 3-D model

experiment3 case5 89.8 23.9 30.41-D model

case6 3-D model

experiment4 case7 29.3

125

17 381-D model

case8 3-D model

experiment5 case9 59.4 16.8 26.81-D model

case10 3-D model

experiment6 case11 89.8 24.2 311-D model

case12 3-D model

experiment7 case13 29.2

200

17.7 37.61-D model

case14 3-D model

experiment8 case15 59.7 22.3 32.21-D model

case16 3-D model

experiment9 case17 90 17.3 23.91-D model

case18 3-D model

Inlet flow rate[L/min.]

1/4 tank

3110mm

900mm

Water surface

OutletSymmetry

1100mm

Inlet

Whole tank

3110mm

1100mm

900mm

Diffuser

Punched-metal

Fig. 7 Analysis domain A-A' section

900mm

Installation depth50mm100mm200mm Tank wall

Symmetry

Inlet

Outlet

Water surface550mm

80mm

40mm

A’A

1555mm

50mm

125mm

550ｍｍ

1/4 tank plan

calculation. 3.3 CFD analysis cases and conditionsThe discharging process for cooling was calculated in non-isothermal unsteady state analysis after the sufficient convergence of flow field in isothermal condition. In the flow visualization experiment of the previous study, three installation depths of diffuser were tested for each three flow rate, i.e., installation depths were 50 mm, 125 mm and 200 mm, and flow rate were 30 L/min, 60 L/min and 90 L/min. All cases of flow rate were investigated for each case of installation depth in the experiment, and two models were applied to simplify the punched metal in the simulation, that is, 18 cases are conducted in total by CFD analysis. All cases are listed in Table 4 and conditions are in Table 5. Water density that is given by quartic function of absolute temperature based on the previous research 5). 3.4 CFD analysis result and discussion Transient processes of vertical temperature distribution of the same installation depth of diffuser (125 mm) are shown in Fig.8 for each cases of flow rate. Those of the same flow rate (60 L/min.) are also shown in Fig.9 for each case of installation depth. According

to these results, as flow rate or installation depth of diffuser is lager, temperature changes more gradually in the case of the 1-dimensional pressure drop model than the experiment. On the other hand, temperature changes almost similarly in the 3-dimensional pressure drop model and the experiment. As the result, the 1-dimensional pressure drop model cannot maintain thermal storage performance specifically at large flow rate and the 3-dimensional pressure drop model showed a good agreement with the experiment compared with the 1-dimensional pressure drop model. 4. ConclusionTo summarize this study, the authors discussed method of punched metal attached to top surface of the diffuser and show the validity of new pressure drop model. It clarified that 3-pressure drop model showed better agreement with the experiment than 1-pressure drop model in thermal storage performance as well as vertical flow behavior observed in the experiment.

REFERENCES1)

2)

3)

4)

5)

This research was supported by MEXT Grant-in-Aid for Scientific Research (B), Sagara.K.,2013

ACKNOWLEDGEMENT

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Fig. 8 Transient processes of vertical temperature distribution for each case of flow rate(Installation depth of diffuser:125mm)

Fig. 9 Transient processes of vertical temperature distribution for each case of installation depth (Flow rate :60 L/min.)

NOMENCLATURE

Iwata, T., Sagara, K. et al., 2012, An Experimental Study on Inflow Behavior of Vertical Inlet for Temperature-stratified Type of Thermal Storage Tank, Proc. Annual meeting of AIJ, 1033-1034 FLUENT12.0/12.1 user's guide, 7.3.2.Porous Jump, (7-167) - (7-168)FLUENT12.0/12.1 user's guide, 7.2.3.Porous Media, (7-22) - (7-394) Yu, Y., Sagara, K. et al., 2010. Numerical Study on Diffuser Shape for Water TES Tank by CFD Analysis, AIJ, Vol.50, 117-120 Niwa, H., Sagara, K., Sekimomo, Y., Inooka, T., 1994.Study on Optimization of Thermal Storage Performance for a Temperature Stratified Thermal Storage Tank,SHASE Transaction, Vol.56, 57-68 (In Japanese)

Δp:the pressure loss (Pa), μ :the viscosity (kg/ms) ρ :the Water density (kg/m3), α : the permeability (m2) C : the inertial resistance coefficient (l/m), v : the velocity (m/s) Δn: the thickness (m)