Enhancing heat transfer surface are used in many engineering applications such as heat exchanger, air conditioning, chemical reactor and refrigeration systems, hence many techniques have been investigated on enhancement of heat transfer rate and decrease the size and cost of the involving equipment especially in heat exchangers. One of the most important techniques used are passive heat transfer technique. These techniques when adopted in Heat exchanger proved that the overall thermal performance improved significantly. This paper reviews experimental and numerical works taken by researchers on this technique since 2004 such as twisted tape, wire coil, swirl flow generator,… etc. to enhance the thermal efficiency in heat exchangers and useful to designers implementing passive augmentation techniques in heat exchange. The authors found that variously developed twisted tape inserts are popular researched and used to strengthen the heat transfer efficiency for heat exchangers. The other techniques used for specific work environments are studied in this paper. Twisted tape inserts perform better in laminar flow than turbulent flow. However, the other several passive techniques such as ribs, conical nozzle, and conical ring, etc. are generally more efficient in the turbulent flow than in the laminar flow.

A comprehensive review on passive heat transfer enhancements in pipe exchangers

Economic reasons (material and energy saving) leads to make efforts for making more efficient heat exchange. The heat transfer enhancement techniques are widely used in many applications in the heating process to make possible reduction in weight and size or enhance the performance of heat exchangers. These techniques are classified as active and passive techniques. The active technique required external power while the passive technique does not need any external power. The passive techniques are valuable compared with the active techniques because the swirl inserts manufacturing process is simple and can be easily employed in an existing heat exchanger. Insertion of swirl flow devices enhance the convective heat transfer by making swirl into the bulk flow and disrupting the boundary layer at the tube surface due to repeated changes in the surface geometry. An effort has been made in this paper to carry out an extensive literature review of various turbulators (coiled tubes, extended surfaces (fin, louvered strip, winglet), rough surfaces (Corrugated tube, Rib) and swirl flow devices such as twisted tape, conical ring, snail entry turbulator, vortex rings, coiled wire) for enhancing heat transfer in heat exchangers. It can be concluded that wire coil gives better overall performance if the pressure drop penalty is considered. The use of coiled square wire turbulators leads to a considerable increase in heat transfer and friction loss over those of a smooth wall tube.

Review of heat transfer enhancement methods: Focus on passive methods using swirl flow devices

Heat transfer enhancement is an important matter of concern for energy conservation and also beneficial from economic point of view. The use of passive devices like twisted tapes, roughness elements, wires inserts etc. are effective methods of heat transfer augmentation. Many research studies on different types of twisted tapes geometries to increase heat transfer rate have been carried out. Also, several correlations were developed to determine heat transfer and friction factor for twisted tape inserts. In the present study, a review on work done in the area of heat transfer augmentation using twisted tapes has been carried out. Previous experimental and numerical studies on various types of twisted tapes (based on the literature survey) were discussed. These studies reveal that the future research in the area of twisted tapes will bring more development in the heat exchanger systems. The optimum shape for twisted tapes can also be developed based on maximisation of heat transfer and minimisation of friction factor regarding fluid used in the system.

Heat transfer augmentation using twisted tape inserts: A review

Investigation of solar collector system with turbulator considering hybrid nanoparticles

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The heat transfer enhancement techniques and their Thermal Performance Factor

The present work shows the results obtained from experimental investigations of the augmentation of turbulent flow heat transfer in a horizontal tube by means of varying width twisted tape inserts with air as the working fluid. In order to reduce excessive pressure drops associated with full width twisted tape inserts, with less corresponding reduction in heat transfer coefficients, reduced width twisted tapes of widths ranging from 10 mm to 22 mm, which are lower than the tube inside diameter of 27.5 mm are used. Experiments were carried out for plain tube with/without twisted tape insert at constant wall heat flux and different mass flow rates. The twisted tapes are of three different twist ratios (3, 4 and 5) each with five different widths (26-full width, 22, 18, 14 and 10 mm) respectively. The Reynolds number varied from 6000 to 13500. Both heat transfer coefficient and pressure drop are calculated and the results are compared with those of plain tube. It was found that the enhancement of heat transfer with twisted tape inserts as compared to plain tube varied from 36 to 48% for full width (26mm) and 33 to 39% for reduced width (22 mm) inserts. Correlations are developed for friction factors and Nusselt numbers for a fully developed turbulent swirl flow, which are applicable to full width as well as reduced width twisted tapes, using a modified twist ratio as pitch to width ratio of the tape. 

International Journal of Engineering, Science and Technology, Vol. 2, No. 6, 2010, pp. 107-118

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Enhancement of heat transfer using varying width twisted tape inserts

The major problem with the direct use of vegetable oils as fuel into CI engines is their higher viscosity. It interferes the fuel injection and atomization and contributes to incomplete combustion, nozzle clogging, excessive engine deposits, ring sticking, producing thick smoke, etc. The problem of higher viscosity of vegetable oils can be overcome to a greater extent by various techniques, such as heating of fuel lines, trans-esterification, modification of injection system, etc. In the present investigation, short term tests were conducted with the use of untreated cotton seed oil in a single cylinder, four stroke, and direct injection diesel engine. Tests were conducted with cotton seed oil and diesel. To improve the combustion characteristics of cotton seed oil in an unmodified engine, effect of increase in injection pressure was studied. The injection pressure was increased from 180 bar to 240 bar (in steps of 15 bar). The investigation revealed that the optimum pressure for cottonseed oil as 210 bar and comparison of the performance of the engine was studied in terms of brake specific fuel consumption, brake thermal efficiency, indicated thermal efficiency, mechanical efficiency and exhaust emissions.

International Journal of Engineering, Science and Technology, Vol. 2, No. 6, 2010, pp. 142-149

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Optimization of injection pressure for a compression ignition engine with cotton seed oil as an alternate fuel

An experimental analysis is done on four stroke single cylinder constant speed, water
cooled diesel engine which is interfaced with Engine soft software. Performance and
emission characteristics are evaluated for three non-edible vegetable oils viz. thumba,
jojoba, neem and jojoba methyl ester to study the effect of injection pressures viz. 205,
220, 240 and 260 bar with variation in injection timing at 23°bTDC and 28°bTDC. The
performance of jojoba methyl ester improved with increase in injection pressures,
maximum brake thermal efficiency of 29.72% is obtained with lower emissions
compared to other vegetable oils; the reason might be low viscosity and better
combustion. Further investigations were carried out with new lubricant SAE 5W-30;
which improved the performance of CI engine by 1.59%. All the above investigations
were fruitful and these results are expected to lead to a substantial contribution to the
development of a viable vegetable oil engine.

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Performance and emission characteristics of a ci engine operated on vegetable oils as alternative fuels

The present work shows the results obtained from experimental investigations of the augmentation of turbulent flow heat transfer in a horizontal tube by means of mesh inserts with air as the working fluid. Sixteen types of mesh inserts with screen diameters of 22mm, 18mm, 14mm and 10mm for varying distance between the screens of 50mm, 100mm, 150mm and 200mm in the porosity range of 99.73 to 99.98 are considered for experimentation. The Reynolds number is varied from 7000 to 14000. Correlations for Nusselt number and friction factor are developed for the mesh inserts from the obtained results. It is observed that the enhancement of heat transfer by using mesh inserts when compared to plain tube at the same mass flow rate is more by a factor of 2 times where as the pressure drop is only about a factor of 1.45 times.

Experimental investigations in a circular tube to enhance turbulent heat transfer using mesh inserts

The present work shows the results obtained from experimental investigations of the augmentation of turbulent flow heat transfer in a horizontal tube by means of varying width twisted tape inserts, with air as the working fluid. In order to reduce excessive pressure drops associated with full-width twisted tape inserts, with a lower corresponding reduction in heat transfer coefficients, reduced-width twisted tapes of widths ranging from 10 mm to 22 mm are used, which are lower than the internal diameter of the tube of 27.5 mm. Experiments were carried out for plain tubes with/without twisted tape inserts at a constant wall heat flux and different mass flow
rates. The twisted tapes are of three different twist ratios (3, 4 and 5), each with five different widths (26 (full-width), 22, 18, 14 and 10 mm). The Reynolds number varied from 6000 to 13500. Both the heat transfer coefficient and pressure drop are calculated, and the results are compared with those of a plain tube. It is observed that with the inclusion of twisted tape inserts there is a significant increase in the heat transfer
coefficients. Correlations are developed for friction factors and Nusselt numbers for a fully-developed turbulent swirl flow, which are applicable to full-width as well as
reduced-width twisted tapes, using a modified twist ratio as the pitch to width ratio of the tape.

/pdf/augmentation-of-turbulent-flow-heat-transfer-in-a-horizontal-1jlfxqobmp.pdf

K. Kalyani Radha

Papers

Enhancement of heat transfer using varying width twisted tape inserts

Optimization of injection pressure for a compression ignition engine with cotton seed oil as an alternate fuel

Performance and emission characteristics of a ci engine operated on vegetable oils as alternative fuels

Experimental investigations in a circular tube to enhance turbulent heat transfer using mesh inserts

Augmentation of Turbulent Flow Heat Transfer in A Horizontal Tube with Varying Width Twisted Tape Inserts