Heat transfer and pressure drop characteristics of forced convective evaporation in horizontal tubes with coiled wire inserts

Heat transfer enhancement and pressure drop increasing during evaporation of R-134a due to the presence of coiled wire insert inside a horizontal evaporator was studied experimentally. The test evaporator was an electrically heated copper tube of 1200 mm length and 7.5 mm inside diameter. Helically wire coils with different wire diameters of 0.5, 0.7, 1.0 and 1.5 mm and different coil pitches of 5, 8, 10 and 13 mm were made and used in full length of the test evaporator. For each inserted tube and also the plain tube, several test runs were carried out with different mass velocities and heat fluxes. From analysis of acquired data, it was found that the coiled wire inserts enhance the heat transfer coefficient but with a higher penalty due to the increasing of pressure drop, in comparison to that for the plain flow. An empirical correlation has been developed to predict the heat transfer coefficient during evaporation inside a horizontal tube in the presence of a coiled wire insert.     

Experimental study of heat transfer enhancement with wire coil inserts in laminar-transition-turbulent regimes at different Prandtl numbers

Helical-wire-coils fitted inside a round tube have been experimentally studied in order to characterize their thermohydraulic behaviour in laminar, transition and turbulent flow. By using water and water–propylene glycol mixtures at different temperatures, a wide range of flow conditions have been covered: Reynolds numbers from 80 to 90,000 and Prandtl numbers from 2.8 to 150. Six wire coils were tested within a geometrical range of helical pitch 1.17 < p/d < 2.68 and wire diameter 0.07 < e/d < 0.10. Experimental correlations of Fanning friction factor and Nusselt number as functions of flow and dimensionless geometric parameters have been proposed. Results have shown that in turbulent flow wire coils increase pressure drop up to nine times and heat transfer up to four times compared to the empty smooth tube. At low Reynolds numbers, wire coils behave as a smooth tube but accelerate transition to critical Reynolds numbers down to 700. Within the transition region, if wire coils are fitted inside a smooth tube heat exchanger, heat transfer rate can be increased up to 200% keeping pumping power constant. Wire coil inserts offer their best performance within the transition region where they show a considerable advantage over other enhancement techniques.     

Numerical evaluation of a heat exchanger with inline tubes of different size for reduced fouling rates

This paper describes the numerical evaluation of a novel cross flow tube bundle heat exchanger that combines tubes of different diameter in an inline arrangement for the purpose of reducing gas side particulate fouling rates while preserving acceptable levels of heat transfer and pressure drop performance. Three arrangements are compared: a common inline tube bundle heat exchanger with cylinders of equal diameter and two other arrangements that consist of alternately placed cylinders with a diameter ratio of d/D = 0.5, at two different transverse spacings. Numerical calculations are performed in order to study heat transfer, pressure drop and fouling rates from flue gases with suspended ash particles. The alternating tube sizes achieve a suppression of the vortex shedding mechanism that has previously been shown to enhance downstream particle deposition. Results show that, compared to the standard arrangement, the tube bundle with unequal cylinders placed at the largest transverse spacing achieves a significant (～30%) reduction in particle deposition rate without sacrificing acceptable values of heat transfer per unit volume and low pressure drop.     

An experimental study of thermal performance of shell-and-coil heat exchangers

In the present study an experimental investigation of the mixed convection heat transfer in a coil-in-shell heat exchanger is reported for various Reynolds and Rayleigh numbers, various tube-to-coil diameter ratios and dimensionless coil pitch. The purpose of this article is to assess the influence of the tube diameter, coil pitch, shell-side and tube-side mass flow rate over the performance coefficient and modified effectiveness of vertical helical coiled tube heat exchangers. The calculations have been performed for the steady-state and the experiments were conducted for both laminar and turbulent flow inside coil. It was found that the mass flow rate of tube-side to shell-side ratio was effective on the axial temperature profiles of heat exchanger. The results also indicate that the ? − NTU relation of the mixed convection heat exchangers was the same as that of a pure counter-flow heat exchanger.     

Enhancement of heat transfer in a tube with regularly-spaced helical tape swirl generators

Influence of helical tapes inserted in a tube on heat transfer enhancement is studied experimentally. A helical tape is inserted in the tube with a view to generating swirl flow that helps to increase the heat transfer rate of the tube. The flow rate of the tube is considered in a range of Reynolds number between 2300 and 8800. The swirling flow devices consisting of: (1) the full-length helical tape with or without a centered-rod, and (2) the regularly-spaced helical tape, are inserted in the inner tube of a concentric tube heat exchanger. Hot air is passed through the inner tube whereas cold water is flowed in the annulus. The experimental data obtained are compared with those obtained from plain tubes of published data. Experimental results confirmed that the use of helical tapes leads to a higher heat transfer rate over the plain tube. The full-length helical tape with rod provides the highest heat transfer rate about 10% better than that without rod but it increased the pressure drop. To overcome this, different free-spacing ratio (s = L_s/L_h) of 0.5, 1.0, 1.5, and 2.0 were examined. It was found that the space ratio value should be about unity for Re < 4000. The regularly-spaced helical tape inserts at s = 0.5 yields the highest Nusselt number which is about 50% above the plain tube.     

On the dynamic detonation parameters in acetylene–oxygen mixtures with varying amount of argon dilution

In this investigation, the dynamic detonation parameters for stoichiometric acetylene–oxygen mixtures diluted with varying amount of argon are measured and analyzed. The experimental results show that the critical tube diameter and the critical energy for direct initiation of spherical detonations increase with the increase of argon dilution. The scaling behavior between the critical tube diameter d_c and the detonation cell size λ as well as the critical direct initiation energy E_c is systematically studied with the effect of argon dilution. The present results again validate that the relation d_c = 13λ holds for 0–30% argon diluted mixtures and breaks down when argon dilution increases up to 40%. It is found that the explosion length scaling of R_o ∼ 26λ becomes also invalid when the mixture contains approximately this same amount of argon dilution or more. This critical argon dilution is indeed close to that found from experiments in porous-walled tubes by Radulescu and Lee (2002) which exhibit a distinct transition in the failure mechanism. Cell size analysis in literature also indicates that the cellular detonation front starts to become more regular (or stable) when the argon dilution reaches more than 40–50%. Regardless of the degree of argon dilution or mixture sensitivity, the phenomenological model developed from the surface energy concept by Lee, which provides a relation that links the critical tube diameter and the critical energy remains valid. The present experimental results also follow qualitatively the observation from chemical kinetic and detonation instability analyses.     

Flow boiling heat transfer characteristics of nitrogen in plain and wire coil inserted tubes

Boiling heat transfer characteristics of nitrogen were experimentally investigated in a stainless steel plain tube and wire coil inserted tubes. Wire coils having different coil pitches and wire thicknesses were inserted into a horizontally positioned plain tube, which had an inner diameter of 10.6 mm and a length of 1.65 m. The coil pitches were 18.4, 27.6, and 36.8 mm, and the wire thicknesses were 1.5, 2.0, and 2.5 mm. Tests were conducted at a saturation temperature of −191 °C, mass fluxes from 58 to 105 kg/m² s, and heat fluxes from 22.5 to 32.7 kW/m². A direct heating method was used to apply heat to the test tube. The boiling heat transfer coefficients of nitrogen significantly decreased when the dryout occurred. Enhancement performance ratio (EPR), which is the ratio of heat transfer enhancement factor to pressure drop ratio, was used to evaluate the performance of the wire coil inserts. The maximum heat transfer enhancement of the wire coil inserted tubes over the plain tube was 174% with wire 3 having a twist ratio (p/D_w) of 1.84 and a thickness ratio (t/D_w) of 0.25. Wire 3-inserted tube showed the highest EPR among the tested tubes in this study.     

Heat transfer properties of nanoparticle-in-fluid dispersions (nanofluids) in laminar flow

The convective heat transfer coefficients of several nanoparticle-in-liquid dispersions (nanofluids) have been measured under laminar flow in a horizontal tube heat exchanger. The nanoparticles used in this research were graphitic in nature, with aspect ratios significantly different from one (l/d ≈ 0.02). The graphite nanoparticles increased the static thermal conductivities of the fluid significantly at low weight fraction loadings. However, the experimental heat transfer coefficients showed lower increases than predicted by either the conventional heat transfer correlations for homogeneous fluids, or the correlations developed from the particle suspensions with aspect ratios close to one. New correlations on heat transfer need to be developed for nanofluid systems.     

Heat transfer augmentation in a helical-ribbed tube with double twisted tape inserts

Turbulent convective heat transfer characteristics in a helical-ribbed tube fitted with twin twisted tapes have been investigated experimentally. The experiment was carried out in a double tube heat exchanger using the helical-ribbed tube having a single rib-height to tube-diameter ratio, e/D_H = 0.06 and rib-pitch to diameter ratio, P/D_H = 0.27 as the tested section. The insertion of the double twisted tapes with twist ratio, Y, in the range of 2.17 to 9.39 is to create vortex flows inside the tube. The inserted ribbed tube is arranged in similar directions of the helical swirl of the twisted tape and the helical rib motion of the tube (called co-swirl). Effects of the co-swirl motion of the ribbed tube and the double twisted tapes with various twist ratios on heat transfer and friction characteristics are examined. The results obtained from the ribbed tube and the twin twisted tape insert are compared with those from the smooth tube and the ribbed tube acting alone. The experimental results reveal that the co-swirling inserted tube performs much better than the ribbed/smooth tube alone at a similar operating condition. The co-swirl tube at Y ≈ 8 yields the highest thermal performance at lower Reynolds number (Re). In addition, the correlations of Nusselt number and friction factor as functions of Re, Pr and Y are also proposed.     

Thermal analysis of solar parabolic trough with porous disc receiver

In this paper, 3-D numerical analysis of the porous disc line receiver for solar parabolic trough collector is presented. The influence of thermic fluid properties, receiver design and solar radiation concentration on overall heat collection is investigated. The analysis is carried out based on renormalization-group (RNG) k–ε turbulent model by using Therminol-VP1 as working fluid. The thermal analysis of the receiver is carried out for various geometrical parameters such as angle (θ), orientation, height of the disc (H) and distance between the discs (w) and for different heat flux conditions. The receiver showed better heat transfer characteristics; the top porous disc configuration having w = d_i, H = 0.5d_i and θ = 30°. The heat transfer characteristic enhances about 64.3% in terms of Nusselt number with a pressure drop of 457 Pa against the tubular receiver. The use of porous medium in tubular solar receiver enhances the system performance significantly.     

Heat transfer characteristics in a tube fitted with helical screw-tape with/without core-rod inserts

Effects of insertion of a helical screw-tape with or without core-rod in a concentric double tube heat exchanger on heat transfer and flow friction characteristics are experimentally investigated. The heat exchanger has the outer and the inner tube diameters of 50 mm (D_o) and 25 mm (D) where the cold and the hot waters used as the test fluids are in shell and tube sides, respectively. The stainless steel helical screw-tape has the geometrical dimensions of width (W) 17 mm with the clearance to the tube wall (D − W) / 2 = 4 mm. Thus, the insertion of the screw-tape in the tube is considered as a loose-fit. In the experiment, the loose-fit helical tape with or without core-rod, is inserted in the inner tube of the heat exchanger and the hot water enters the tube based on its Reynolds number in a range of 2000 to 12,000. The experimental results show that the increases in average Nusselt number of using the loose-fit, helical tape with and without core-rod are found to be 230% and 340%, respectively, over the corresponding plain tube. It is worth noting that for the loose-fit, helical tape without core-rod, the friction factor is around 50% less than that for the one with core-rod while the Nusselt number is about 50% higher. Furthermore, the enhancement efficiency of the helical screw-tapes varies between 1.00 and 1.17, 1.98 and 2.14, for the tapes with and without core-rod, respectively.     

Conjugate heat transfer and thermal stress analysis of wire coil inserted tubes that are heated externally with uniform heat flux

Wire coil inserted tubes are important in engineering applications. The conjugate heat transfer and thermal stress, which is induced by temperature differences in the wire coil inserted tube sheet of heat transfer equipment, were studied numerically in the paper. Three different wire coil inserts, which have different pitches, are considered. The smooth tube is also considered for comparison. Uniform heat flux was applied from the external surface of the tube. Water has been used as fluid. The energy and governing flow equations were solved using a finite difference scheme. The finite element method (FEM) was used to compute the thermal stress fields. The effect of the different wire coils on the thermal stress has been discussed in terms of the results extracted from the FEM. It was found that the maximum thermal stresses ratio occurred in the case of p = 2d for 3 m/s mean water velocity. Some designs to reduce the thermal stress in the wire coiled tube sheet were suggested.     

Two-phase friction factor in vertical downward flow in high mass flux region of refrigerant HFC-134a during condensation

The two-phase pressure drop of the pure refrigerant HFC-134a during condensation inside a vertical tube-in-tube heat exchanger was investigated. The double tube test section was 0.5 m long with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube was constructed from smooth copper tubing of 8.1 mm inner diameter and 9.52 mm outer diameter. The test runs were performed at average condensing temperatures of 40–50 °C. The mass fluxes were between 260 and 515 kg m^− 2 s^− 1 and the heat fluxes between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section was calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section was directly measured by a differential pressure transducer. A new correlation for the two-phase friction factor of R134a flow is proposed by means of the equivalent Reynolds number model. The effects of heat flux, mass flux and condensation temperature on the pressure drop are also discussed.     

Turbulent convection in round tube equipped with propeller type swirl generators

This article is aimed at studying the heat transfer, friction loss and enhancement efficiency behaviors in a heat exchanger tube equipped with propeller type swirl generators at several pitch ratios (PR). The investigation is performed for the Reynolds number ranging from 4000 to 21,000 under a uniform heat flux condition. The experiments are also undertaken for several blade numbers of the propeller (N = 4, 6, and 8 blades) and for different blade angles (θ = 30°, 45°, and 60°). The influences of using the propeller rotating freely, on heat transfer enhancement, pressure loss, and enhancement efficiency, are reported. In the experiments, the swirl generator is used to create a decaying swirl in the tube flow. Average Nusselt numbers are determined and also compared with those obtained from other similar cases. The experimental results indicate that the tube with the propeller inserts provides considerable improvement of the heat transfer rate over the plain tube around 2.07 to 2.18 times for PR = 5, blade angles θ = 60° and N = 8. The use of the propeller leads to maximum enhancement efficiency up to 1.2. Thus, because of strong swirl or rotating flow, the propellers and their blade numbers become influential upon the heat transfer enhancement. The increase in friction factor from using the propeller is found to be 3–18 times over the plain tube. Correlations for Nusselt number (Nu) and friction factor (f) for the inserted tube are provided and the performance evaluation criterion to access the real benefits in using the swirl generators of the enhanced tube is also determined.     

Experimental investigation of convective heat transfer and pressure loss in a round tube fitted with circular-ring turbulators

This paper presents the effect of the circular-ring turbulator (CRT) on the heat transfer and fluid friction characteristics in a heat exchanger tube. The experiments were conducted by insertion of CRTs with various geometries, including three different diameter ratios (DR = d/D = 0.5, 0.6 and 0.7) and three different pitch ratios (PR = p/D = 6, 8 and 12). During the test air at 27 °C was passed through the test tube which was controlled under uniform wall heat flux condition. The Reynolds number was varied from 4000 to 20,000. According to the experimental results, heat transfer rates in the tube fitted with CRTs are augmented around 57% to 195% compared to that in the plain tube, depending upon operating conditions. In addition, the results also reveal the CRT with the smallest pitch and diameter ratios offers the highest heat transfer rate in accompany with the largest pressure loss.     

Convective heat transfer in a circular tube with short-length twisted tape insert

This work presents an experimental study on the mean Nusselt number, friction factor and enhancement efficiency characteristics in a round tube with short-length twisted tape insert under uniform wall heat flux boundary conditions. In the experiments, measured data are taken at Reynolds numbers in a turbulent region with air as the test fluid. The full-length twisted tape is inserted into the tested tube at a single twist ratio of y/w = 4.0 while the short-length tapes mounted at the entry test section are used at several tape length ratios (LR = l_s/l_f) of 0.29, 0.43, 0.57 and 1.0 (full-length tape). The short-length tape is introduced as a swirling flow device for generating a strong swirl flow at the tube entry before decaying along the tube. On the other hand, the full-length tape (LR = 1.0) is expected to produce a strongly swirling flow over the whole tube. The variation of heat transfer and pressure loss in the form of Nusselt number (Nu) and friction factor (f) respectively is determined and depicted graphically. The experimental result indicates that the short-length tapes of LR = 0.29, 0.43 and 0.57 perform lower heat transfer and friction factor values than the full-length tape around 14%, 9.5% and 6.7%; and 21%, 15.3% and 10.5%, respectively. In addition, it is apparent that the enhancement efficiency of the tube with the short-length tape insert is found to be lower than that with the full-length one. The mean deviation between measured and correlated values of the Nusselt number is in the order of ± 7% in the range of Reynolds numbers from 4000 to 20,000.     

Performances of a heat exchanger and pilot boiler for the development of a condensing gas boiler

In this research, design factors for a heat exchanger and boiler were investigated using a simplified model of a heat exchanger and pilot condensing boiler, respectively. Specifications of each heat exchanger component (e.g., upper heat exchanger (UHE) and lower heat exchanger (LHE); coil heat exchanger (CHE); baffles) were investigated using a model apparatus, and the comprehensive performance of the pilot gas boiler was examined experimentally. The heating efficiency of the boiler developed was about 90% when using the optimal designed heat exchangers. Compared to a conventional Bunsen-type boiler, the heating efficiency was improved about 10%. Additionally, NO_x and CO emissions were about 30 ppm and 160 ppm, respectively, based on a 0% O₂ basis at an equivalence ratio of 0.70, which is an appropriate operating condition. However, the pollutant emission of the boiler developed is satisfactory considering the emission performance of a condensing boiler, even though CO emission must be reduced.     

Energy and exergy analyses of an externally fired gas turbine (EFGT) cycle integrated with biomass gasifier for distributed power generation

Biomass based decentralized power generation using externally fired gas turbine (EFGT) can be a technically feasible option. In this work, thermal performance and sizing of such plants have been analyzed at different cycle pressure ratio (r_p = 2−8), turbine inlet temperature (TIT = 1050–1350 K) and the heat exchanger cold end temperature difference (CETD = 200–300 K). It is found that the thermal efficiency of the EFGT plant reaches a maximum at an optimum pressure ratio depending upon the TIT and heat exchanger CETD. For a particular pressure ratio, thermal efficiency increases either with the increase in TIT or with the decrease in heat exchanger CETD. The specific air flow, associated with the size of the plant equipment, decreases with the increase in pressure ratio. This decrease is rapid at the lower end of the pressure ratio (r_p < 4) but levels-off at higher r_p values. An increase in the TIT reduces the specific air flow, while a change in the heat exchanger CETD has no influence on it. Based on this comparison, the performance of a 100 kW EFGT plant has been analyzed for three sets of operating parameters and a trade-off in the operating condition is reached.     

Effect of coil-wire insert on heat transfer enhancement and pressure drop of the horizontal concentric tubes

In the present study, the heat transfer characteristics and the pressure drop of the horizontal double pipe with coil-wire insert are investigated. The inner and outer diameters of the inner tube are 8.92 and 9.52 mm, respectively. The coiled wire is fabricated by bending a 1 mm diameter of the iron wire into a coil with a coil diameter of 7.80 mm. Cold and hot water are used as working fluids in the shell side and tube side, respectively. The test runs are performed at the cold and hot water mass flow rates ranging between 0.01 and 0.07 kg/s, and between 0.04 and 0.08 kg/s, respectively. The inlet cold and hot water temperatures are between 15 and 20 °C, and between 40 and 45 °C, respectively. The effect of the coil pitch and relevant parameters on heat transfer characteristics and pressure drop are considered. Coil-wire insert has significant effect on the enhancement of heat transfer especially on laminar flow region. Non-isothermal correlations for the heat transfer coefficient and friction factor are proposed. There is reasonable agreement between the measured data and predicted results.     

Estimates of pressure gradients in PEMFC gas channels due to blockage by static liquid drops

Numerical analyses are presented to explain the effect of drop size and contact angle on local pressures inside small channels. These pressures and channel characteristics are of interest when water condenses in the gas channels of Proton Exchange Membrane Fuel Cells and hence the study uses Reynolds numbers consistent with as typical utilization of reacting gases in 200 cm² flow fields (i.e., 200 < Re < 1500 and stoichiometries of 1.2–2.0 at 1.0 A/cm²). The analyses were performed using three-dimensional computational fluid dynamic techniques and the results show that pressure drops are minimal until the blockage was greater than 50%. As blockage increased further, due to larger drops or increased hydrophobicity, pressure drop increased. The results of a stagnant drop are supported by visualization experiments, which show minimal distortion of the drop for these low flow rates, small ratios, and hydrophobic contact angles. Proper scaling parameters and design criteria for microchannels validation experiments are presented.