Experimental Study on the Heat transfer of MPCM Slurry as a Heat‐transfer Fluid

International Journal of Energy and Power (IJEP) Volume 4, 2015 www.ijep.org doi: 10.14355/ijep.2015.04.009

Experimental Study on the Heat transfer of MPCM Slurry as a Heat‐transfer Fluid Shan lv*1, Mei He2, Pan Zhang3, Zhongzhu Qiu4 School of Energy and Mechanical Engineering, Shanghai University of Electric Power, 2588 Changyang Road, China *1

lvshan9983@163.com; 2hemei@163.com; 3zhanxin@163.com; 4zhzhu@163.com

Abstract The experiment of turbulent heat convection of microencapsulated PCM slurry which flows in a circular tube was based on the MPCM slurry. The slurry used isopropanol/water mixed solution as the base fluid and MPCM28 as the phase change microcapsule particles. The results showed that the pressure drop of heating pipe increased with the increase of turbulent intensity, and decreased with the increasing temperature. However, Nux of the MPCMS increased with the increasing Re as well as the concentration of slurry. But increased first and decreased then with the increase of heat flux.

Keywords MPCM Slurry; Turbulent Convection; Drag Reduction

Introduction With the rapid development of technology and severer issue of energy and environment, the problems concerning the energy transfer, metallurgy, chemistry and so forth have raised much research interest. However, because of the limited ability of heat transfer, the conventional fluids are not able to meet the real demands. So as to fulfil the requirements, the functional fluids such as the phase change fluid come into the field of thermal energy research [1‐2]. Compared with conventional fluids, the phase change fluids can increasingly enhance the efficiency of heat transfer, which is an effective method to realize the energy utilization with high efficiency.

Massive of researches about the characteristics of heat convection with different kinds of phase change fluid have been conducted by many researchers [3‐6]. Wang Li [7] designed and constructed the experiment apparatus for turbulent heat transfer in the tube which used micro‐encapsulated phase change suspension as working fluid.

Zhao Bingquan et al [8] upgraded Wang’s experiments. Rhafiki et al [9] built a physical model of heat convection for the functional phase change fluid on the condition of constant heat flux. Huang Yong et al [10] established a physical model of heat convection for the functional phase change fluid in the horizontal circular tube applied magnetic field. Huang analysed the influence of different factors on heat convection as well.

This paper re‐designed and constructed the experiment rig for turbulent heat transfer in the tube based on previous research. It further explored the pressure drop and heat convection efficiency of microencapsulated phase change suspension during turbulent heat convection.

Experiment Apparatus and Methodology The experiment apparatus which is consist of three circulation loops shows as FIG. 1. In the first loop circuit, microencapsulated phase change slurry was heated in the horizontal section of the heating pipe which was pumped from the open water tank, then entered to the plate heat exchanger for cooling, the cooling medium was water which was from the second loop circuit. It aimed at reducing the temperature of the MPCMS, and then backed into the open water tank for recycling. Both the working fluid of the second and third loop circuit was water. Besides, we also used water source heat pump to ensure its good cooling effect, that is to say, cooled the MPCMS down to the melting point below to ensure the first loop sustained and effective.

The test section was a stainless steel tube in horizontal. The material of the tube was the 304 stainless steel, and its size was 3.7m long, 4mm internal diameter, 6mm external diameter. The electric heating belt which has the thickness of 1.5mm and width of 15mm is uniformly wound on the test section of the lateral. In order to reduce the

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heat loss, the appearance of bread was wrapped in refractory fiber insulation cotton. Fluid temperature inside the tube was measured by high fever rate silicon thermocouple, the measured data was read by the digital CENTER300 which measured accuracy is ±0.1℃. The surface of stainless steel was arranged 8 thermocouples, the inlet and outlet position had its own thermocoulples, the others were distributed on the heating pipe. Because the power MPCMS needed to become turbulent, it was impossible to merely rely on the gravity or use high nitrogen pressure to achieve the effect. We chose to use centrifugal pump whose lift is 8m to drive the flow of pipeline to become turbulent. The measuring range of float flowmeter was 0‐300L/h, vertical placement.

Injected the MPCMS into open water tank, it was heated on the test section with the power of centrifugal pump until the system condition reaches to a stable thermal equilibrium. The data such as flux, the pressure of inlet and outlet and temperature should be recorded. Different conditions can be made through changing flux or heat flux.

When the outlet temperature and pressure values of horizontal heated tube stay stability (no more than 0.2℃ and 0.1kpa), the test system has reached thermal equilibrium, test data should be recorded.

In this paper, the MPCMS used isopropanol/water mixed solution as the base fluid which has the density of 0.936g/ml and MPCM28 as the phase change microcapsule particles which has the appreance of white powder, phasetransition temperature was 28℃, latent heat of phase change was 180‐195J/g. By altering the adding quality of microcapsule particle, eventually made the concentration of MPCMS respectively 5wt.% and 10wt.%, which can stay stability for up to 48 h .

FIG. 1 SCHEME OF EXPERIMENTAL APPARATUS

Results and Discussion The Effects of Reynolds and Temperature on the Pressure Drop The pressure drop values of single‐phase and mass concentration of 5wt.% and 10wt.% MPCMS were increasing with the turbulent intensity increases as FIG. 2 shown. It meaned that the MPCMS has smaller flow resistance than the base fluid. With the Reynolds increases, the MPCMS has better fluidity. This was mainly because microencapsulated phase particles have inhibitory effect on turbulence.

The curves of Reynolds change are shown in FIG. 3. One of the situations is constant heat flux, and the other is at normal atmospheric temperature. The curves showed that in the heat flux of 400w/m2, the concentration of 5wt.% MPCMS‘s flow resistance increases with the turbulent intensity increases. But, compared to the pressure drop of normal atmospheric temperature, pressure drop values seemed much lower. This was mainly because the MPCMS’s viscosity reduced with the increasing of temperature, thereby the resistance reduces, the pressure drop

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decreases. From the two types of pressure drop curves, we know that the pressure drop values increase with the increasing of turbulent intensity, and decrease with the temperature increases.

120 Single Phase Fluid 5wt%MPCMS 10wt%MPCMS

Pressure Drop(pa)

100 80 60 40 20 0

2000

3000

4000

5000

6000

7000

8000

9000

Re

FIG. 2 THE INFLUENCE OF REYNOLDS ON PRESSURE DROP FIG. 3 THE INFLUENCE OF HEATING CONDITIONS

The Influence of Reynolds on Turbulent Heat Transfer The relationship of Reynolds and Nux is showed in FIG. 4. The curves involved 3 different types of fluids‐5wt.% MPCMS ,10wt.% MPCMS and single phase fluid. The single phase fluid which becomes fully developed flow can be calculated accurately according to Gnielinski Correlation [11] in the condition of Reynolds between 2300 and 10000. The error between theoretical value and experimental value is less than 8%. The experimental values of single phase fluid are higher than the theoretical values, this is mainly due to the Gnielinski Correlation caused by improper use since the correlation is for fully developed turbulent flow. Both of the 5wt.% and 10wt.% MPCMS have higher convective heat transfer coefficients than the single phase fluid, which is 1.2‐2.46 times larger than the single phase fluid. This is due to the turbulent process, inhibition of microencapsulated particles.

Gnielinski Correlation: Nu x 

 Re-1000） Pr （f / 8） （ , 1  12.7( f / 8)1/ 2 (Pr 2/3  1)

f  (1.82 log Re  1.64) 2 (1) 70 60 50 Nu

40 30 20

Gnielinski（1976）

10

5wt%

0 2000

Single phase fluid 10wt%

4000

Re

6000

8000

FIG. 4 THE RELATIONSHIP OF Nu AND REYNOLDS

The Influence of Concentration of MPCM on Turbulent Heat Transfer The relationship of Nux and non dimensional axial is shown in FIG. 5. In the condition of 400w/m2 heat flux density and 1.3m/s velocity, the Nux of single phase fluid can be calculated according to Churchill Correlation [12].

Compared to the single phase fluid, the Nux of 10wt.% MPCMS has improved about 60%. So the concentration of the MPCMS is a key factor which influences the Nux. With the increase of mass concentration, the disturbance of microcapsuled particles increases, so it leads to the enhancement of the MPCMS.

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Churchill Correlation: Nu x  5.364[1 （

200 x*



）10/9 ]3/10  1.0

x* （x / d）（ / Re1b Pr1b ） (2)

Where d – Internal diameter; x‐the diatance between entrance and test point The Influence of Heat Flux on Turbulent Heat Transfer In order to explore the influence of heat flux on the turbulent heat transfer, 5wt.% and 10wt.% MPCMS have been selected to do the research. FIG. 6 and FIG. 7 indicate that in the different heating conditions which refer to 300w/m2, 400w/m2 and 500w/m2, the Nux of MPCMS increases firstly and then decreases.

FIG. 5 THE INFLUENCE OF CONCENTRATION ON Nu

FIG. 6 THE INFLUENCE OF DIFFERENT HEATING FIG. 7 THE INFLUENCE OF DIFFERENT HEATING CONDICTIONS ON Nu (5wt.%) CONDICTIONS ON Nu (10wt.%)

Conclusions In this paper, we mainly studied the influence of turbulent intensity, mass concentration, heating conditions on the turbulent heat transfer. The experiments provided the following conclusions.

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1.

The addition of microencapsule particles have significant inhibitory effect on the turbulence. With the Re‐ increases, the resistance of MPCMS reduces. In the experimental conditions, when the heat flux increases, the flow resistance reduces.

2.

The theoretical values of 5wt.%, 10wt.% and single phase fluid are less than the experimental values which are been calculated according to Gnielinski Correlation. But with the increase of Reynolds, the turbulent heat transfers of 5wt.% and10wt.%MPCMS are better than the single phase fluid.

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3.

Reynolds has positive effects on the MPCMS in turbulent flow. With the increase of Reynolds, Nu increases.

Turbulent process exacerbates the microencapsulated phase change material particles on the turbulent inhibition. Meanwhile, in the same experimental condition, the enhancement effect of heat transfer is mainly related to the concentration of the suspension, the bigger the mass concentration, the bigger the Nu is. But Nu is influenced by heat flux, and the trend increases first then decreases.

ACKNOWLEDGMENT

The authors gratefully acknowledge Professor Qiu for all kinds of support. This work is financially supported by Shanghai Engineering Research Centre of Power Generation Environment Protection (11dz2281700) and the Key Fund of Shanghai Science Technology Committee (13160501000).

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Shan. Lv was born in Hubei in 1990. Lv is a master student of Shanghai University of Electric Power since the year of 2013 whose research interests are microencapsuled phase change materials suspension, heat convection .

She has several published articles, for instance, “Supercooling suppression of Microencapsulated PCM”, “The influence of drag reduction on the stability and viscosity of MPCMS” and so forth. Her current research focuses on the stability of MPCMS and biomass and biogas Cogeneration.

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