Non-dimensional analysis of experimental pressure drop and energy dissipation measurements in Oscillatory Baffled Reactors

An experimental study is performed to characterize the pressure drop and the power consumption in Oscillatory Baffled Reactors, using dimensionless numbers: the oscillatory Fanning friction factor (f osc ) and the Power number (Po), respectively. Two baffle geometries (one-orifice and three orifices) are tested, for different fluids and oscillating amplitudes. The range of oscillatory Reynolds numbers (Reosc ) tested is 10–1000. Data reduction based on the statistical fitting and the FFT of the pressure drop and velocity signals is introduced to assess the maximum pressure drop and the phase lag between both signals. The new set of experimental data proves the limitations of the conventional models available in the open literature. f osc and Po provide consistent dimensionless results for the different working fluids tested and their trends are clearly related to different flow behaviours: laminar or chaotic flow. Correlations for f osc and Po as a function of Reosc and dimensionless amplitude are obtained.The authors gratefully acknowledge the financial support of the project DPI2015-66493-P by Ministerio de Economía y Competitividad (MINECO, Spain) and the European Regional Development Fund (ERDF)

The effect of velocity ratio on thermal-hydraulic performance of reciprocating scraped surface heat exchangers at low Reynolds number

The thermal-hydraulic performance of a reciprocating scraper inserted in a round tube at low Reynolds number is studied. Pressure drop and heat transfer characteristics have been experimentally determined in static conditions in laminar regime (Reh=30), and results are contrasted with dynamic performance at several velocity ratios (ω=0.1 - 1). Maximum increases of Fanning friction factor of the order of 1.2 have been found, together with increases in Nusselt number of the order of 2, using propylene-glycol as working fluid.This research has been partially financed by the DPI2007-66551-C02-01 grant of the ''Dirección General de Investigación del Ministerio de Educación y Ciencia de España'' and the “HRS Spiratube” company

Flow field and heat transfer investigation in tubes of heat exchangers with motionless scrapers

Flow pattern and thermal-hydraulic characteristics in an innovative tube insert have been experimentally and numerically investigated. The insert device is a concept envisioned for reciprocating scraped surface heat exchangers. It consists of a concentric rod, that mounts an array of semicircular plugs fitted to the inner tube wall. In motionless conditions, the insert works as a turbulence promoter, enhancing heat transfer in laminar regime. Fundamental flow features in the symmetry plane of the tube have been assessed with Particle Image Velocimetry technique. A general model of the flow mechanism has been defined, identifying three regions along a geometrical pitch: recirculation bubbles, flow acceleration and transverse vortex. Results have been complemented with experimental data on pressure drop and heat transfer. The transition onset is clearly identified, and the mechanisms that promote turbulence at low Reynolds number are investigated and discussed. CFD simulations for different Reynolds numbers provide a further insight into the relation of the flow structures with wall shear stress, and their role on the local heat transfer augmentation

Performance evaluation of a zero-fouling reciprocating scraped surface heat exchanger

An innovative self-cleaning shell and tube heat exchanger is presented. Inside each interior tube (through which the product flows) a scraping rod is fitted. This rod moves in reciprocal manner and the scraping elements mounted on the rod fully clean the tube wall surface. Additionally, the macroscopic displacements of the flow, induced by the insert device motion, promote high flow mixing. Consequently, tube-side heat transfer coefficients are enhanced. Thermal-hydraulic and scraping power measurements are performed in laminar regime for 20<Reh<250 and 0≤ω≤1 (ω=uscr/uf). An extended Performance Evaluation Criterion is proposed, in order to balance the augmentations of heat transfer and the increased power consumption (pumping and scraping power) of the device. This study allows stating guidelines for the operation of the device, concluding that the performance of the heat exchanger is irrespective of the velocity ratio. The scraper can be used intermittently, or at the minimum scraping frequency that ensures fouling mitigation

Performance evaluation of a zero-fouling reciprocating scraped surface heat exchanger

An innovative self-cleaning shell and tube heat exchanger is presented. Inside each interior tube (through which the product flows) a scraping rod is fitted. This rod moves in reciprocal manner and the scraping elements mounted on the rod fully clean the tube wall surface. Additionally, the macroscopic displacements of the flow, induced by the insert device motion, promote high Flow mixing. Consequently, tube-side heat transfer coefficients are enhanced. Thermal-hydraulic and scraping power measurements are performed in laminar regime for 20<Reh<250 and 0≤ω≤1 (ω=uscr/uf). An extended Performance Evaluation Criterion is proposed, in order to balance the augmentations of heat transfer and the increased power consumption (pumping and scraping power) of the device. This study allows stating guidelines for the operation of the device, concluding that the performance of the heat exchanger is irrespective of the velocity ratio. The scraper can be used intermittently, or at the minimum scraping frequency that ensures fouling mitigation

Experimental correlations on critical Reynolds numbers and friction factor in tubes with wire-coil inserts in laminar, transitional and low turbulent flow regimes

This paper analyses 23 circular helicoidal wire-coils with different geometric characteristics ranging from: dimensionless pitch p/d = [0.25–3.37], dimensionless thickness e/d = [0.071–0.286] and a Reynolds number interval from 50 to 8000. This interval widely includes the Reynolds number range in which rigid wire-coil inserts present better performance as passive enhancement technique for tubular heat exchanger applications Re = [200–2000]. Based on their hydraulic performance, the wire-coil inserts are categorized according to a new dimensionless parameter: the Transition Shape Parameter (TSP). A new set of correlations are obtained to predict the Fanning friction factor coefficient as a function of Reynolds number and geometrical characteristics of the insert within the three flow regimes: laminar, transitional and low turbulent. Additional correlations are proposed to estimate the critical Reynolds number at the beginning and ending of the transition region, which allows to select the most adequate friction factor correlation as a function of the operational Reynolds number for a heat exchanger design application. Finally, a comparative between the proposed and the published correlations in the open literature for laminar and turbulent regimes is presented. This brings to light the need and interest of having the suitable and reliable set of correlations presented in this paper to compute the friction coefficient covering all the wire-coil applicability range as an enhancement technique.The authors gratefully acknowledge the “Fundación Séneca” (Fundación Séneca: Project with Ref. 15297/PI/10) and the Spanish Ministry of Science (Project with Ref. ENE2011-28571-C02-01) for supporting this research

Validation of a new methodological approach for the selection of wire-coil inserts in thermal equipment

The use of wire-coils is especially relevant at low Reynolds numbers (below the critical number to turbulent flow in smooth tubes) according to its inherent positive features such as the advance of transition onset and, if they present suitable geometric characteristics, the establishment of an extended transitional flow in a critical Reynolds number interval [ReCL − ReCT ], with a predictable friction coefficient and Nusselt number. This paper presents the experimental validation of a new methodology based on the evaluation of a non-dimensional geometry-based parameter: the TSP (Transition Shape Parameter) that allows to predict the friction coefficient evolution with wire-coil inserts and enables to compute the extension of the transitional flow region. The close relationship between hydraulic and thermal performance of wire-coil inserts makes this methodology a valuable tool for selecting the most appropriate wire-coil geometry for a given tubular heat exchanger. It is observed that to promote an increase in heat transfer, the value of the ReCL of the wire coil must be less than the operating Reynolds number range of the equipment. Thus, the ReCL − ReCT interval of the insert should fall into this range. In order to validate the methodological approach, an application to harp-type solar thermal collectors with typical Reynolds number range [40–6000] is presented. Four representative wire-coils, with a wide geometrical range characterized by TSP values of 759, 196, 35.3 and 3.1 (exhibiting significant differentiated behaviours in their friction factor curves and critical Reynolds numbers) were inserted inside the risers of a modified solar collector and experimentally tested at laboratory conditions. Static temperature at different locations at the absorber plate, and pressure drop were measured to obtain friction factor and Nusselt number inside riser covering the laminar, transitional and low-turbulent regions. For a general application with friction factor constraints the most suitable wire-coil geometry is the TSPW02 = 196 with a range of critical Reynolds number of ReCL = 663 and ReCT = 2286 and Nuw02/Nus = 2.21 for Re = [300 − 3000] with f w02/fs = 3, 82. However, for the case study presented (a harp-type solar collector) it is feasible to insert the third wire-coil geometry TSPW03 = 35.3 due to its early transition, with a range of critical Reynolds number of ReCL = 364 and ReCT = 2324, and Nuw/Nus = 1.35 for Re = 300 with a high friction factor augmentation f w03/fs = 18.84. This geometry also promotes the highest absorber temperature reduction. The greatest temperature reduction is observed in the range of Reynolds numbers [700–2000], reaching approximately 6 ◦C, which represents approximately 15 %The authors gratefully acknowledge to European Regional Development Fund and Ministerio de Ciencia e Innovación - Agencia Estatal de Investigación for the financial support of the project ALTES: “Active Latent Thermal Energy Storage”, Ref. PGC2018-100864-B-C21

The role of insert devices on enhancing heat transfer in a flat-plate solar water collector

This work presents a comparative experimental study of heat transfer enhancement in a flat-plate solar water collector using insert devices. Three wire-coils and three twisted-tapes were selected with representative geometrical characteristics typically employed in industrial applications. Isothermal pressure drop tests were carried out to obtain the fully-developed Fanning friction factor for a range of Reynolds numbers Re = [80–9000]. The increase in friction factor in comparison to smooth tube was computed for all the devices. Depending on Reynolds number and insert geometry fi/fs values ranged from 1.3 to 79.8. Furthermore, detailed temperature profiles were obtained for different sections along the absorber plate and the risers for five different mass flow rates covering the Reynolds range from [400–2500]. The increase of the inner heat transfer coefficient by the inserts caused an important decrease of the absorber temperature. At increasing mass flow rates (from Re ≈ 1000), all the inserts showed a very similar thermal performance which make them suitable for inserting within harp-type solar collectors, where pressure drop is not a constraint. The best inserts TT03, WC01 and WC02 gave at Re ≈ 1500 maximum absorber temperature decreases (insert vs smooth tube) of 5.05 °C, 5.40 °C and 5.34 °C. In serpentine-type solar collectors, due to pressure drop constraints, the wire coil WC01 with a moderate pitch to wire-diameter ratio (p/d = 1.5 and e/d = 0.07), is the best specimen to insert. WC01 presents a moderate pressure drop increase (fi/fs = 2.8 at Re ≈ 1000), an early promotion of turbulent flow (at Re ≈ 700), and a significant reduction of the absorber temperature (decreasing 4.84 °C vs smooth tube at Re ≈ 1000).The authors gratefully acknowledge the “Agencia de Ciencia y Tecnología de la Región de Murcia” (Fundación Séneca: Project with Ref. 15297/PI/10) and the Spanish Ministry of Science (Project with Ref. ENE2011-28571-C02-01) for supporting this research

Experimental investigation of turbulence level in enhanced heat exchangers with active insert devices

This work presents a visualization study carried-out on a dynamic insert device. The flow pattern is obtained by employing the Particle Image Velocimetry (PIV) technique. The insert device is moved alternatively along a tube and consists of several circular elements with six circumferentially distributed holes on them, which are mounted on a shaft with a pitch of 5D. The whole is moved alternatively along the axial direction by a hydraulic cylinder. The increase of the turbulence level of the Flow will be analyzed and related to the heat transfer augmentation. By the use of Particle Image Velocimetry technique and water as test fluid, the 2-Dimensional pattern of the turbulent flow is obtained on the two symmetry planes of the device: hole center and between holes. The results permit to establish the flow pattern along the devices. In static conditions of the scraper, experiments are carried out at three different Reynolds ranging from 4000 to 6323. In dynamic conditions, the Reynolds number has been kept constant at 7400, while varying the velocity of the scrapper in relation of 0.5, 1 and 2 with the bulk velocity of the flow. Co current and counterflow directions of the scraping device have been analyzed, and results show the insert device movement on flow behavior

Analysis of heat transfer phenomena during ice slurry production in scraped surface plate heat exchangers

Heat transfer during ice slurry production in a scraped surface plate heat exchanger (SSPHE) has been experimentally investigated. By using a 7 wt. % sodium chloride brine, a wide range of operating conditions has been tested: scraping velocities from 0.1 to 0.8 s−1 and logarithmic temperature differences from 0.5 to 2.5 °C. Two different PEEK scraper arrangements have been used, mounted on the driving arms: rigid scrapers and surface adaptable scrapers, pushed by torsion springs. Heat transfer coefficients and ice production rate were measured under batch operating mode. Experimental results shown dependence of the nucleation onset with the scraping speed and the wall supercooling degree. Global nucleation only occurred for high velocities and low supercooling degrees, appearing only on the wall for the other cases. A decrease of the heat transfer coefficient of 1.5 times for increasing logarithmic temperature differences is reported, as a consequence of the ice layer growth with a low effect of the scraping speed. The use of adaptable scrapers provide heat transfer coefficient augmentations from 2 to 4-fold with respect to the rigid configuration