 Original Paper
 Open Access
The top heat mode of closed loop oscillating heat pipe with check valves at the top heat mode (THMCLOHP/CV): a thermodynamic study
 Nipon Bhuwakietkumjohn^{1} and
 Thanya Parametthanuwat^{1}Email author
https://doi.org/10.1186/s4071201400058
© Parametthanuwat et al.; licensee Springer 2014
 Received: 13 February 2014
 Accepted: 13 May 2014
 Published: 7 August 2014
Abstract
The article reports a recent study on heat flux of the top heat mode closed looped oscillating heat pipe with check valves (THMCLOHP/CV). An experimental system was evaluated under normal operating conditions. The THMCLOHP/CV was made of a copper tube with an inside diameter of 2.03 mm. The working fluid was water, ethanol and R123 with a filling ratio of 30%, 50% and 80% with respect to the total volume of the tube. The angles of inclination were 20°, 40°, 60°, 80° and 90° from the horizontal axis. The number of turn was 40 turns and 2 check valves. Three lengths of evaporator investigated were 50, 100 and 150 mm. The operating temperatures were 45°C, 55°C and 65°C. Experimental data showed that the THMCLOHP/CV at evaporator length of 50 mm gave a better heat flux with filling ratio at 50% when using R123 as working fluid and the operating temperature of 65°C at angles of inclination of 90°. It was further found that an evaporator length of 50 mm was superior in heat flux over other length in all experimental conditions under this study. Moreover, the presence of operating temperature had clearly contributed to raise the heat flux of THMCLOHP/CV, but the heat flux had decreased when evaporator length increased.
Keywords
 Top heat mode (THM)
 Oscillating heat pipe (OHP)
 Check valve (CV)
Background
There are indications that exploratory research are indeed required to study the filling ratio, working fluid, length of evaporator section, operating of temperature and angle of inclination of these on top heat mode in engineering systems. Furthermore, this article aims to study heat flux behaviour of THMCLOHP/CV.
Methods
Check valves
Experimental setup
Controlled and variable parameters
Controlled and variable parameters  Values 

The variable parameters  L _{ e } 50, 100 and 150 mm 
Filling ratio of 30% 50% and 80% (by total volume)  
Working temperatures of 45°C, 55°C and 65°C  
Working fluid of R123, ethanol and water  
Inclinations angles of 20°, 40°, 60°, 80° and 90°  
The controlled parameters  Number of check valves, 2 
Air inlet of 0.6 m/s  
Number of turns 40 turns  
Tube inner diameter of 2.03 mm 
Uncertainty analysis result
Quantity source of uncertainty  Value of quantity  Uncertainty type  Confidence level  Converge factor  Standard uncertainty  Sensitivity coefficient  Uncertainty component  Combined uncertainty  Expanded uncertainty 

(k )  (u _{ i } )  (c _{ i } )  (u _{ i } c _{ i } )  (u _{ c } )  (U )  
Temperature measurement  
Thermocouple type K, °C  −270 to 1,372  Type B  95%  2  0.57735  1  0.57735  0.33362  0.66724 
Data logger, °C  −200 to 1,100  Type B  95%  2  0.57735  1  0.57735  
Uncertainty of mean reading, °C    Type A  95%  2  0.024  1  0.024  
Flow measurement  
Air flow meter, m/s (Operation range, −30°C to 140°C)  0 to 20  Type B  95%  2  0.12  1  0.12     
Results and discussion
Effect of inclination angles on heat flux
Effect of filling ratio on heat flux
Effect of operating temperature on heat flux
Effect of working fluid on heat flux
Their properties are different with respect to their density, surface tension and latent heat of vapourization (Reay and Kew 2006; Dobson 2004). Concerning these three properties, the latent heat of vapourization is the major property that has the greatest effect on the motion of the liquid slugs and vapour bubbles in a tube, as well as in the heat transfer rate of the HCLOHP/CVs of Rittidech et al. (2010). Therefore, if the working fluid changes from water and ethanol to R123, the heat flux increases; these are shown in Figures 4, 5, 6. This may be because of R123 that has a low latent heat of vapourization, as well as the fact that the boiling point of R123 is lower than those of the the water and ethanol (Dunn and Reay 1982; Incropera and Dewitt 1996; Bhuwakietkumjohn and Rittidech 2010). The boiling point is an important parameter for the THMCLOHP/CV working temperature and performance. If the boiling point is low, the THMCLOHP/CV will work at low temperatures (Rittidech et al. 2010; Thongdaeng et al. 2012; Koito et al. 2009). However, the latent heat and boiling point of the working fluid also has an effect on THMCLOHP/CV performance.
Effect of evaporator length on heat flux
Conclusions

The filling ratio had a slight effect on thermal performance of the THMCLOHP/CV. The thermal performance of the THMCLOHP/CV with L _{ e } of 50 mm was higher than the L _{ e } of 100 and 150 mm at a filling ratio of 50% when using R123 as working fluid.

The operating temperature had an effect on the heat flux of the THMCLOHP/CV; when the operating temperature was increased, the heat flux increased.

The angle of inclination of THMCLOHP/CV affected the heat flux because of the gravitational head. It depended on fluid density, acceleration from gravity force and the length of tube.

As the L _{ e } increases from 50 to 150 mm, the heat flux slightly decreases. The longer L _{ e } had occurred, the boiling phenomenon approaches pool boiling, and at pool boiling, a low heat flux occurs.

It was further found that the physical properties (filling ratio, L _{ e }, angle of inclination and operating temperature) had effect on the ratio of heat transfer rates in normal operation, but the properties of the working fluid affected the heat transfer rate.
Abbreviations

A _{ c }, all outer surface area of tube, m ^{ 2 }

Q, heat transfer rate, W

q, heat flux (W/m^{2})

\( \overset{\cdotp }{m} \), mass flow rate, kg/s

C _{ p }, specific heat capacity constant pressure, J/kg°C

T, temperature,°C

Fr, filling ratio, %

L _{ e }, length of evaporator, mm

d _{ i,max}, inner diameter of copper tube, mm

D _{ o }, outside diameter, m

g, gravitational acceleration, m/s^{2}

σ, surface tension, N/m

ρ, density of fluid, kg/m^{3}

_{in}, inlet

_{out}, outlet
Declarations
Acknowledgements
Generous support from the Faculty of Industrial and Technology Management through Department of Design and Production Technology of Agricultural Industrial Machinery (Grant No. FITM560200415) to this research is acknowledged. Thanya Parametthanuwat and Nipon Bhuwakietkumjohn were also supported generously by Sampan Rittidech, head of the HeatPipe and Thermal Tools Design Research Unit (HTDR), Faculty of Engineering, Mahasarakham University; Thailand, and Thailand Research Fund and Office of The Higher Education Commission.
Authors’ Affiliations
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