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Increasing importance of heat transfer in chemical engineering science causes that investigation in the field of enhancement techniques is always one of the up-to-date topics for study. In the current comparative analysis, the thermal enhancement and friction penalty are explored numerically for curved tubes via twisted configuration. To accomplish this, three common geometries namely helical, serpentine, and Archimedes spiral, are considered at different coil-pitches and twist-pitches as well as five Reynolds numbers in the laminar flow regime. The results exhibit noticeable enhancements (up to 60%) in the thermal performance of the twisted cases as compared to the smooth cases. The highest increases are recorded for the serpentine case, followed by the helical and spiral cases. It is found that these enhancements vary via coil-pitch and twist-pitch. Increasing coil-pitch and twist-pitch augments both heat transfer coefficient and pressure drop in all curved-twisted tubes, however, the effects of twist-pitch are more pronounced. To predict Nusselt number and friction factor, new correlations are also proposed. The maximum deviations of the predicted results compared to the simulated data are within ±5%.

In this investigation, the combined use of twisted tubes and nanofluid (NF) to augment the performance of a double-pipe heat exchanger (DPHE) has been considered numerically. Steady-state laminar flow of the cold non-Newtonian CuO NF and hot water pass inside the tube side and annulus side, respectively. The base fluid is the aqueous solution of 0.5 mass% carboxymethyl cellulose. The effects of the Reynolds number ( $${\text{Re}}$$ ), the volume concentration of nanoparticles ( $$\varphi$$ ) and twist pitch on the performance metrics are examined, and the outcomes are compared with those a plain DPHE. The outcomes showed that the increase in Re has desirable effects such as improved heat transfer and heat exchanger effectiveness, and unpleasant effects such as increased pressure drop and pumping power. Moreover, it was found that except for $$\varphi \le 1.5\%$$ and $${\text{Re}} =$$ 500, the NF performs better than the base fluid. In addition, it was reported that the variation pattern of overall hydrothermal performance of NF with twist pitch is ascending–descending. Furthermore, the outcomes illustrated that the overall hydrothermal performance of twisted DPHE is superior to that of the plain DPHE, and its highest value is 2.671, which belongs to case of $${\text{Re}} =$$ 2000, $$\varphi =$$ 3% and twist pitch = 4 mm.

Natural gas is transported from producing regions to consumption regions by using transmission pipelines at high pressures. At consumption regions, the pressure of natural gas is reduced in city gate stations (CGSs). Before the pressure reduction process, the temperature of natural gas is increased usually by using a water bath heater, which burns natural gas as fuel, to protect against freezing of natural gas. These types of heat exchangers have a low efficiency and consume a lot of fuel to generate the required heat. In the current study, the twisted configuration of the heating coil is proposed and investigated to enhance the heat transfer through a water bath heater with a nominal capacity of 1000 m3·h−1. Firstly, the implementation procedure is validated with data collected from the CGS of Qaleh-Jiq (located in Golestan province of Iran). A very good agreement is achieved between the obtained results and the real data. Then, three different twist ratios are considered to examine the twisting effects. The proposed technique is evaluated in the terms of velocity, temperature, and pressure variations, and the results are compared with the conventional case, i.e. straight configuration. It is found that both the heat transfer rate and the pressure drop augment as the twist ratio is raised. Finally, it is concluded that the twisted tubes can reduce the length of the gas coil by about 12.5% for the model with low twist ratio, 18.75% for the model with medium twist ratio, and 25% for the model with high twist ratio as compared to the straight configuration.

Background The use of food waste as feedstock shows high production of biogas via anaerobic digestion, but requires efficient heat transfer in food waste slurry at heating and cooling processes. The lack of rheological properties hampered the research on the heat-transfer process for food waste slurry. Referentially, the twisted hexagonal and elliptical rubes have been proved as the optimal enhanced geometry for heat transfer of medium viscous slurries with non-Newtonian behavior and Newtonian fluids, respectively. It remains unknown whether improvements can be achieved by using twisted geometries in combination with food waste slurry in processes including heating and cooling. Results Food waste slurry was observed to exhibit highly viscous, significant temperature-dependence, and strongly shear-thinning rheological characteristics. Experiments confirmed the heat-transfer enhancement of twisted hexagonal tubes for food waste slurry and validated the computational fluid dynamics-based simulations with an average deviation of 14.2%. Twisted hexagonal tubes were observed to be more effective at low-temperature differences and possess an enhancement factor of up to 2.75; while twisted elliptical tubes only exhibited limited heat-transfer enhancement at high Reynolds numbers. The heat-transfer enhancement achieved by twisted hexagonal tubes was attributed to the low dynamic viscosity in the boundary layer induced by the strong and continuous shear effect near the walls of the tube. Conclusions This study determined the rheological properties of food waste slurry, confirmed the heat-transfer enhancement of the twisted hexagonal tubes experimentally and numerically, and revealed the mechanism of heat-transfer enhancement based on shear rate distributions.

Additional file 1: Table S1. Tested data of rheological properties for FWS with TS = 10%. Table S2. Numerical results of FWS in THT. Table S3. Numerical results of FWS in TET. Table S4. Numerical results of FWS in CT

Segmental baffles are used in the conventional shell and tube heat exchanger (STHX). Helical baffles can be used in place of segmental baffles, which increase the heat transfer efficiency and lessen the pressure losses. To avoid flow induced vibrations and eliminate stagnant recirculation zones, a new type of clamping anti-vibration baffle is proposed. Three dimensional CFD simulations, have been performed on ANSYS FLUENT to study and compare the pressure drop and heat transfer coefficient between the newly designed clamping anti-vibration baffles with square twisted tubes, helical baffles with cylindrical tubes and conventional segmental baffle with cylindrical tubes. For the numerical comparison whole heat exchangers are modeled, the numerical model shows the thermo-hydraulic performance with good accuracy by comparing it with Kern and Esso methods for single segmental baffles. It is then used to compare the performance of the same heat exchanger with clamping anti-vibration baffles with square twisted tubes (CBSTT-STHX) and helical baffles with cylindrical tubes (HBCT-STHX). CBSTT-STHX has a higher heat transfer coefficient than segmental baffles with cylindrical tubes (SGCT-STHX) and less pressure drop than both helical baffle with cylindrical tube (HBCT-STHX) and segmental baffle with cylindrical tubes (SGCT-STHX), while its comprehensive performance is higher than SGCT-STHX and slightly less than HBCT-STHX.

IF=2.471; International audience; Numerical simulations were performed with the open-source CFD software OpenFOAM to investigate the ability of several configurations of short-length twisted tube geometries with non-circular cross section connected to tubes with circular cross section to induce a swirling flow. The heat transfer and the pressure drop linked to the generated swirling flow are also calculated. The swirling flow is modeled using a k-ω SST turbulence model with a low-Reynolds approach. It is shown that a short-length twisted tube with an elliptical cross section (STE) is able to generate a swirling flow, but its intensity greatly depends on its twist pitch and its aspect ratio. The lower the aspect ratio, the higher the swirl intensity. For a Reynolds number ranging from 10,000 to 100,000, the results reveal that compared to a plain tube, the STE with the lowest aspect ratio achieves enhancing the heat transfer from 22 to 90% at the cost of an increased pressure drop of, respectively, 63 and 129%. The second part of the study is focused on a short-length twisted tube with a three-lobed cross section, and the results reveal that the generated swirling flow is even more intense than with the STE and that the heat transfer enhancement goes from 30 to 105% at the cost of an increased pressure drop from 137 to 180%.

The thermal and hydraulic performance of a twisted circular tube with an obstacle attached to the internal surface along the tube length (L = 1000 mm) is numerically analyzed in this work. The considered configuration is investigated by varying obstacle height ratio (OHR = h/D) which is the obstacle height (h) to the tube diameter (D = 20 mm) while obstacle thickness is kept constant under turbulent flow regime, i.e. Reynolds number 4000 ≤ Re ≤ 10,000 for air as working fluid. Various cases of OHR are triggered 0.0, 0.125, 0.25, 0.375, 0.5 and 0.625 respectively. The results show that the larger OHR gives a higher Nusselt number and vice versa due to increasing swirling power to the air in the tube. However, a higher friction factor is the penalty associated with this large obstacle. Recorded thermal performance factor TPF is beyond unity for all cases. The optimal value of TPF is observed at OHR = 0.625 which is recommended for high heat transfer rate requirement. Corresponding TPF = 2.07 at Re = 4000 and TPF = 1.69 at Re = 10,000. The results are validated against the findings of the previous works and the comparisons show good agreements.

We consider a three-dimensional waveguide that is a small deformation of a periodically twisted tube (including in particular the case of a straight tube). The deformation is given by a bending and an additional twisting of the tube, both parametrized by a coupling constant $$\delta $$. In this deformed waveguide, we consider the Dirichlet Laplacian. We expand its resolvent near the bottom of its essential spectrum, and we show the existence of exactly one resonance, in the asymptotic regime of $$\delta $$ small. We are able to perform the asymptotic expansion of the resonance in $$\delta $$, which in particular permits us to give a quantitative geometric criterion for the existence of a discrete eigenvalue below the essential spectrum. In the case of perturbations of straight tubes, we are able to show the existence of resonances not only near the bottom of the essential spectrum but near each threshold in the spectrum, showing in particular what are the spectral effects of the bending for higher energies. We also obtain the asymptotic behavior of the resonances in this situation, which is generically different from the first case.

We show that a twist of a three-dimensional tube of uniform cross-section yields an improved decay rate for the heat semigroup associated with the Dirichlet Laplacian in the tube. The proof employs Hardy inequalities for the Dirichlet Laplacian in twisted tubes and the method of self-similar variables and weighted Sobolev spaces for the heat equation., Comment: 35 pages, LaTeX with 2 EPS figures; a misprint in Conjecture on page 33 corrected