Heat Transfer Characteristics through Rifled Tubes

A Review on Heat Transfer Characteristics through Rifled Tubes

Keywords: Mass flux, Rifled tubes, Heat transfer coefficient, Critical heat flux.

Abstract. Rifled tubes are gaining more attention to researchers due to improved performance in heat transfer. Rifled tubes also known as spiral internally ribbed tube is a rough surface tube.This enhances heat transfer coefficient even when the mass flux is less. In this paper the heat transfer characteristics such heat transfer coefficient, critical heat flux, wall temperature distribution and thermal equilibrium of horizontal and vertical rifled tubes under different flows are studied and analyzed. Based on that suggestions and recommendations are provided for experimental investigation.

Introduction

Evaporation of water into steam in boilers is done using rifled tubes. The internal rifling allows improved heat transfer between the tube and the fluid. Recent research is conducted to improve the heat transfer efficiency. Rifled or ribbed tubes are used to improve the heat transfer efficiency of heat exchangers by introducing centrifugal force which separates water and steam and preventing the formation of steam film. Hence the surface area is improved which maximizes the heat transfer.

This improvement in heat transfer is due to increase in internal surface area. Boiling crisis can occur due to sudden increase in internal surface area. The efficiency of rifled tubing is more under two-phase flow conditions. The internal surface area percentage is higher for evaporator coil in two-phase flow than does a condenser with its superheating and sub-cooling sections. Many heat exchangers like condensers, evaporators, boilers used in various commercial and industrial applications use heat transfer enhancement techniques[1] for increasing heat transfer. There are three categories of heat enhancement techniques like active, passive and compound. These techniques increases convective heat transfer coefficient with or without increase in surface area.

The rifled tube causes a swirling effect in the flow and also due to inner geometry shape a secondary flow called helical flow occur at the tube periphery. This enhances wall wetting and prevents occurrence of critical heat transfer even under high steam. The operating condition of rifled tube is high compared to smooth tube. In order to improve tube cooling at low-mass flux a new type rifled tubes with special rib configurations are used. Fig.1 shows the cut section of the rifled tube test specimen. Rifled tube has internal helical ribs with inner diameter, rib height, rib width, helical angle, outer diameter. Rifled tube is made of copper which has more thermal conductivity which is good for experimentation. Important advantage of rifled tube is the swirl flow created by the fluid flowing along the helical ribs which increases the heat transfer. The helically shaped rifled tubes have some effect on the redistribution of heat which affects the boiling.

Previous Work

The turbulent heat transfer and temperature profiles are experimentally measured in a rifled pipe by Smith, J. W. et. Al [1].The author constructed the spiral rib with copper bar.It has a pitch to diameter ratio of 2.58 which is placed inside a smooth brass tube. From the experimental results the temperature profiles indicates decrease in heat transfer resistance. Heat transfer efficiency is improved when compared to smooth tube.

Fig.1 Rifled Tube

The heat transfer characteristics in the near critical pressure region was studied by Iwabuchi, M. et al[2]for rifled tubes.The rifled tube was constructed with 4 ribs and had an inner diameter of 17.7mm ,rib height of 0.83mm and 30^o helix angle. From the experiment result it is found that even in the sub cooled region CHF (Critical Heat Flux) condition appears due to less swirl effect.

The effect of internal surface rifling on heat transfer characteristics in internally ribbed tubes was investigated by Köhler, W. & Kastner, and W [3]. The experimental parameters like pressure (50-220 bar), mass velocity (500-1500 kg/m2s), and heat flux (0-600kW/m2) were considered for experimental investigation. The author made the rifled tube with 4 ribs, 25.4 mm outer diameter, and 0.775mm rib height and 58^o helix angle. Due to internal rifling boiling crisis and higher post Critical Heat Flux (CHF) occur when compared to smooth tube. From the results it is seen that boiling crisis had occurred due to internal rifling and wall temperature is reduced when compared to smooth tube. It is also seen that due to swirl flow thermal equilibrium occured in the unwetted region.

The local and average transport coefficient for the turbulent flow in rifled tubes was investigated by Almeda, J. A. & Souza Mendes, P. R[5].The authors found that the friction factor is highly sensitive when the rib height is H/D and is not sensitive when the pitch is P/H. Heat transfer coefficient is more with Reynolds number if H/D is less than 0.02 and vice versa. But the authors failed to investigate the pumping power with heat transfer.

The heat transfer characteristics like frictional pressure drop in a two-phase flow vertical rifled tube and smooth tube was analyzed by Cheng, L. X. & Chen, T. K. [8] under the condition of 0.6 MPa. The dimensions of rifled tube used are 22mm of outside diameter, 5.5mm of rib width, 0.4mm - 0.6mm of rib height and rib pitch 3.5mm.The experimental results shows that the superheat wall temperatures are less than that of the smooth tubes for the same heat and mass fluxes.

The critical heat flux (CHF) of R134a is evaluated by C. H. Kim et al. [9] under different flows in a vertical smooth and rifled tube that is heated uniformly. The CHF enhancement is 40% - 60% more than that of the smooth tube. The experimental results show that the critical helical angle and critical velocity also affects the CHF enhancement along with mass flux and pressure.

Experimental analysis on heat transfer characteristics like heat transfer coefficient and pressure drop due to friction in a single phase rifled tube and smooth tube was investigated by Cheng, L. X. & Chen, T. K [10].Water and kerosene are considered as working fluids. Heat transfer characteristics of rifled tubes are compared with that of smooth tubes. The result shows that both the parameters were improved for rifled tubes when compared to that of the smooth tube for water and for kerosene.

Cheng, L. X. et.al had investigated the vertical rifled tube for pressure drop [6] under two-phase conditions .It uses Friedel model for experimentation and a two-phase homogeneous model. Pressure drop from the specified two models are validated during experimentation. Results shows that the pressure drop due to friction in the rifled tube was greater when compared to that of the smooth tube.

The post dry-out with R-134a in smooth tube and rifled tube using upward flow was experimentally studied by Lee, S. K. & Chang, S. H.Three types of rifled tube with 4 head, 60ohelix angle and 17.04 mm maximum inner diameter. The authors have compared the effect of rib geometry with smooth tube having 17.04 inner diameter and 22.59 outer diameter. Results show that the wall temperature is very low when compared to smooth tube.

The low mass flux rifled tube was experimentally investigated by Jie Pan et.al [4]for heat transfer characteristics with upward flow. An investigation for heat transfer characteristics in a vertical rifled tube using water under low mass flux was performed. The wall temperature distribution in the rifled tube at various conditions is obtained and the good heat transfer performance indicates that the low mass flux rifled tube can provide adequate tube cooling and ensure safe operation of the boiler water wall.

Proposed System

The heat transfer characteristics of rifled tubes are to be tested with various fluids such as water, air, oil, CO2 etc with low mass flux density. The experimental setup is shown in fig.2. The heat transfer coefficient, wall temperature distribution and thermal equilibrium are to be analyzed and compared with smooth tube.

T- Temperature measuring point, P- Pressure measuring point

Fig.2 Test Section of the System

Fig.2 shows the test section of the system with an inner tube made of copper with smooth continuous tube and the same is replaced by rifled tube for comparative experimentations. Hot fluid is circulated inside the inner rifled tube. The flow rate is measured using flow meter. The temperature and pressure at various nodes are measured with the help of thermocouple and pressure gauges respectively .Experimental investigations are to be done with working fluids of constant mass flux which is circulated inside rifled tube.

Fig.3 Schematic diagram of the proposed system

The experimental setup for the proposed research is shown in Fig.3 .Hot Fluid is circulated using a pump through the inner rifled tube and the cold fluid is circulated outside the tube with the help of another pump .The flow rates of both hot and cold fluid are measured using flow meters. Temperature distribution across the test section and at the inlet and outlet are measured using RTD’s (thermocouple).Heat transfer coefficient is expectedly more in rifled than the smooth tube. Scaling factor may affect the efficiency of rifled tube.

Conclusion

This paper concludes that the heat transfer increases in rifled tubes than the smooth tubes due to swirl flow. Many authors have proved this in their investigations. The rifling with different dimensions such as tube diameter, pitch, rib height, rib width, helical angle are the factors affecting the heat transfer. There is scope for investigations with modified rifling which is a good area for research. This paper recommends more experimental investigations in the field of heat transfer through rifled tubes.

References

1. Smith, J. W. et al., Turbulent Heat Transfer and Temperature Profiles in a Rifled Pipe,Chemical Engineering Science. 23(1968) 751-758.
2. Iwabuchi, M. et. al., Heat Transfer Characteristic of Rifled Tubes in the Near Critical Pressure Region, Proceeding of 7th International Heat Transfer Conference. 5(1982) 313-318.
3. Köhler, W. and Kastner, W. Heat transfer and pressure loss in rifled tube,Proceeding of 8th International Heat Transfer Conference. 5(1986) 2861- 2865.
4. Jie Pan, Dong Yang, Zichun Dong, Ton Zhu, Qincheng Bi, Experimental Investigation on heat transfer characteristics of low mass flux rifled tube with upward flow, International Journal of heat and mass transfer.(2011)2952-2961.
5. Almeida, J. A. and Souza Mendes, P. R. Local and Average Transport Coefficients for the Turbulent Flow in Internally Ribbed Tubes, Experimental Thermal and Fluid Science. 5(1992) 513-523.
6. Cheng, L. X. & Chen, T. K, Study of vapour liquid two-phase frictional pressure drop in a vertical heated spirally internally ribbed tube, Chemical Engineering Science. 62(2007) 783-792.
7. Lee, S. K. et. al., Experimental Study of Post-Dryout with R-134a Upward Flow in Smooth Tube and Rifled Tubes, International Journal of Heat and Mass Transfer. 51(2008) 3153-3163.

8. L.X. Cheng, T.K. Chen, Flow boiling heat transfer in a vertical spirally internally ribbed tube, Heat and Mass Transfer. 37(2001) 229-236.

9. Kim, C. H. et al., Critical heat flux performance for flow boiling of R-134a in vertical uniformly heated smooth tube and rifled tubes. International Journal of Heat and Mass Transfer 48(2005) 2868-2877.

10. Cheng, L. X. and Chen, T. K, Study of single phase flow heat transfer and friction pressure drop in a spiral internally ribbed tube. Chemical Engineering Technology.29 (2006) 588-595.