Multi-tube System Research Articles

Thermal energy storage with PCMs: A comprehensive study of horizontal shell and multi-tube systems with finned design

Due to its unique qualities and advantages, phase change materials (PCMs) are crucial for thermal energy storage in various applications. The poor conductivity of PCMs is known to cause delayed charging and discharging rates. So, various techniques were proposed by the researchers to improve the thermal performance of PCM systems. To address this challenge, a comprehensive numerical research was presented on a horizontally oriented finned shell-and-tube type of PCM storage. The investigation was divided into four parts: (i) effects of the quantity and arrangement of heat transfer fluid (HTF) tubes; (ii) effects of the quantity and arrangement of fins; (iii) effects of the length of fins; and (iv) effects of various configurations of the finned shell-and-tube PCM storage. The outcomes depicted that due to buoyancy impact and horizontal location, the upper portion of the PCM storage melted more quickly than the lower portion. Also, it was found that, concerning the effect of the HTF arrangement (lacking fins) on the PCM melting process, with a 67.1 % improvement factor over a base model without fins, a double vertical array of HTF tubes produced the quickest melting time. Moreover, regarding evaluating the impact of the fins arrangement (one HTF tube), with a 39 % improvement factor, the diamond arrangement of fins was shown to be the best configuration for speeding up melting. For the complete melting process (LF = 1), the most significant improvement factor of 53.2 was noted for a fin length of 5.1 mm. Finally, the obtained numerical outcomes depicted that the complete melting time (at LF = 1) was reduced by 69.14 % in the model with a double vertical array of HTF tubes and a double vertical arrangement of fins, making it the ideal layout for the finned shell-and-tube PCM storage system.

Eccentricity analysis of annular multi-tube storage unit with phase change material

Thermal energy storage (TES) with phase change materials (PCM) can improve energy efficiency by reducing energy availability and demand mismatch. However, despite the potential of this technology, the low thermal conductivity of PCM slows down the storage cycle and limits its commercialisation. The literature has proposed many solutions to improve heat transfer, such as fins, encapsulation, or metal foam inserts, which are too expensive and complicated. This study focuses on an inexpensive and practical solution, the eccentricity of shell and tube heat exchanger (STHE). Eccentricity's impact has been well established, but its effects on multi-tube cases have not yet been examined. This study investigates the eccentricities of four horizontal STHE with 1, 2, 3, and 4 tubes. The paper presents the ideal combinations of tube number and eccentricity for maximising TES efficiency and explores the underlying heat transfer and fluid mechanics phenomena. The enthalpy double porosities model accurately predicts the PCM melting and solidifying characteristics with varied eccentricities. Results reveal that the eccentric charge performance is better than the concentric tube(s) arrangement. The optimal eccentricity of the charging cycle depends on the tube number. The optimal eccentricities of the single and multi-tube systems are 6 and 10. The total melting time of optimal eccentricity for the four-tube, three-tube, two-tube, and single-tube is optimised by 63 %, 63 %, 60 % and 54 % compared to the concentric cases, respectively. The optimal eccentricity of the single-tube case has the shortest melting time, which increases linearly with the Rayleigh number. The concentric cases consistently exhibit the best performance and shortest solidification time as the Rayleigh number increases. The total solidification time of the single-tube case is reduced by 56 %, 52 % and 49 % compared to the four-tube, three-tube and two-tube, respectively. The multiphysical mechanisms of the charge and discharge cycles are discussed.

Two-phase Flow Dynamics at the Interface Between GDL and Gas Distributor Channel Using a Pore-Network Model

For improved operating conditions of a polymer electrolyte membrane (PEM) fuel cell, a sophisticated water management is crucial. Therefore, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. Pore-scale modeling of diffusion layers and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the cathode layers, a particular challenge is the combination and interaction of the multi-phase flow in the porous material of the gas diffusion layer (GDL) with the free flow in the gas distributor channels. The formation, growth and detachment of water droplets on the hydrophobic, porous surface of the GDL have a major influence on the mass, momentum and energy exchange between the layers. A dynamic pore-network model is used to describe the flow through the porous GDL on the pore-scale. To capture the droplet occurrence and its influence on the flow, this dynamic two-phase pore-network model is extended to capture droplet formation and growth at the surface of the GDL as well as droplet detachment due to the gas flow in the gas distributor channels. In this article, the developed model is applied to single- and multi-tube systems to investigate the general drop behavior. These rather simple test-cases are compared to experimental and numerical data available in the literature. Finally, the model is applied to a GDL unit cell to analyze the interaction between two-phase flow through the GDL and drop formation at the interface between GDL and gas distributor channel.

Assessment of Heat Transfer Characteristics of a Latent Heat Thermal Energy Storage System: Multi Tube Design

In this study, a numerical investigation of heat and mass transfer characteristics of a high temperature phase change material (PCM) is presented. The phase change material used is Sodium Nitrate with a melting point of 306 °C. For a fixed PCM mass, investigations are carried out on a multi-tube heat exchanger system for examining the effects of tube size and number of tubes on overall heat transfer rate. The performance of latent heat storage system during the charging and discharging processes are discussed. Further, the results are elaborated for three multi-tube systems to optimize the number of tubes to be used with the constant heat transfer surface area. The natural convection pattern during charging is observed with the help of temperature contours and streamlines. The solidification front pattern of the PCM, during the discharging process is visualized through melt fraction contours. With the increase in the number of tubes from 13 to 25, the heat distribution throughout the storage system is very effective which lead to reduction in charging and discharging times by 20% and 48% respectively.

Engineering small tubes with changes in diameter for the study of kidney cell organization.

Multicellular tubes are structures ubiquitously found during development and in adult organisms. Their topologies (diameter, direction or branching), together with their mechanical characteristics, play fundamental roles in organ function and in the emergence of pathologies. In tubes of micrometric range diameters, typically found in the vascular system, renal tubules or excretory ducts, cells are submitted to a strong curvature and confinement effects in addition to flow. Then, small tubes with change in diameter are submitted to a local gradient of shear stress and curvature, which may lead to complex mechanotransduction responses along tubes, and may be involved in the onset or propagation of cystic or obstructive pathologies. We describe here a simple method to build a microfluidic device that integrates cylindrical channels with changes in diameter that mimic in vivo tube geometries. This microfabrication approach is based on molding of etched tungsten wires, which can achieve on a flexible way any change in diameter in a polydimethylsiloxane (PDMS) microdevice. The interest of this biomimetic multitube system has been evidenced by reproducing renal tubules on chip. In particular, renal cell lines were successfully seeded and grown in PDMS circular tubes with a transition between 80 μm and 50 μm diameters. Thanks to this biomimetic platform, the effect of the tube curvature has been investigated especially regarding cell morphology and orientation. The effect of shear stress on confluent cells has also been assessed simultaneously in both parts of tubes. It is thus possible to study interconnected cell response to differential constraints which is of central importance when mimicking tubes present in the organism.

The use of enhanced heat transfer phase change materials (PCM) to improve the coefficient of performance (COP) of solar powered LiBr/H2O absorption cooling systems

The underlying principle employing latent heat storage to store excess solar energy to drive solar powered absorption cooling system and space heating/hot water system has been experimentally investigated. The experimental setup investigated the performance of a 4.5 kW absorption system (COP of 0.66) and a PCM system. Four PCM experiments were conducted using 20 kg each of Erythritol to compare 3 heat transfer enhancement techniques; circular fins, longitudinal fins and multitube systems. The multitube system performed best with shortest complete melt time of 275 min (longitudinal finned – 430 min; circular finned incomplete melting within 8 h). The multitube system however, experienced subcooling during discharge. The longitudinal finned system performed best during discharge and experienced no subcooling. Overall utilization efficiencies achieved for multitube and longitudinal finned systems were 83.2% and 82% respectively. PCM quantities that will be required to meet 4 h of 100% peak cooling load for a semi-detached house (105 m2) were determined to be 242 kg and 238 kg respectively. To ensure overall improved charge and discharge performance, a combination of multitube and longitudinally finned in some configuration would be required. Modified equations for the evaluation of improved COPs of a LiBr/H2O absorption system have been proposed.

Rapid determination of tritium and carbon-14 in urine samples using a combustion technique

Traditionally in bioassay monitoring, 3H determinations in urine have been performed using either direct counting (with or without sample decolourisation) or distillation whilst 14C has been determined on separate sub-samples following chemical isolation of the carbon from samples. Although these techniques are relatively straightforward they do not permit simultaneous determination of 3H/14C. For 3H, direct counting can be significantly affected by variations in sample composition/colour resulting in varying degrees of sample quench, does not distinguish between aqueous/organically-bound 3H and is limited to small sample sizes. This study describes the use of purpose built multi-tube combustion furnace for the simultaneous extraction and determination of 3H and 14C. The technique is insensitive to sample composition and can be adapted to measure Tritiated water (HTO) and organically bound tritium separately. The development of a multi-tube system with integrated cool-down facility permits rapid throughput of high sample numbers and has been proven effective in decommissioning waste characterisation. In addition, the furnace-based technique is capable of processing larger sample sizes, increasing analytical sensitivity and accuracy of dose assessment.

Experimental study of a multi-tube ICS solar water heating system in mild climates

Since a majority of residential and industrial building hot water needs are around 50°C, an integrated solar water heater could provide a bulk source that blends collection and storage into one unit. These collectors incorporate thermal storage within the collector itself. The storage pipe surface serves as the absorber surface. Most ICS systems use only one tank, but some use a number of tanks in series. While the simplicity of ICS systems is attractive, they are generally suitable only for applications in mild climates with small thermal storage requirements. A multi-tube ICS solar hot water system with eight cylindrical water storage tanks (pipes) in horizontal (East-West) orientation was designed and outdoor tests of experimental model were performed in mild climate of north (Caspian Sea) region of Iran. Experimental results such as water temperature profiles, mean daily efficiency, water temperature stratification and thermal losses during night are presented and discussed for this model. Experimental results showed that the daily mean efficiency is comparable to other ICS systems and also acceptable thermal performance of this type of ICS system has been observed.

Heat transfer enhancement in medium temperature thermal energy storage system using a multitube heat transfer array

An experimental energy storage system has been designed using an horizontal shell and tube heat exchanger incorporating a medium temperature phase change material (PCM) with a melting point of 117.7 °C. Two experimental configurations consisting of a control unit with one heat transfer tube and a multitube unit with four heat transfer tubes were studied. The thermal characteristics in the systems have been analysed using isothermal contour plots and temperature time curves. Temperature gradients along the three directions of the shell and tube systems; axial, radial and angular directions have been analysed and compared. The phase change in the multitube system was dominated by the effect of convective heat transfer compared to conductive heat transfer in the control system. The temperature gradient in the PCM during phase change was greatest in the radial direction for both the control and multitube systems. The temperature gradients recorded in the axial direction for the control and multitube systems during the change of phase were respectively 2.5 and 3.5% that of the radial direction, indicating essentially a two-dimensional heat transfer in the PCM. The onset of natural convection through the formation of multiple convective cells in the multitube system significantly altered the shape of the solid liquid interface fluid flow and indicates the requirement for an in-depth study of multitube arrangements.

3224 曲がり多重チューブ(CMTS)を用いた制御型手術支援システム(J06-6 スマートシステム・デバイス,J06 知的材料・構造システム)

3224 曲がり多重チューブ(CMTS)を用いた制御型手術支援システム(J06-6 スマートシステム・デバイス,J06 知的材料・構造システム)

Development of 10-kWe preferential oxidation system for fuel cell vehicles

A preferential oxidation (PROX) reactor for a 10-kWe polymer electrolyte membrane fuel cell (PEMFC) system is developed. Pt-Ru/Al 2O 3 catalyst powder, with a size of 300–600 μm is applied for the PROX reaction. To minimize pressure drop and to avoid hot spots in the catalyst bed, the reactor is designed as a dual-staged, multi-tube system. The performance of the 10-kWe PROX unit is evaluated by feeding simulated gasoline reformate which contains 1.2 wt.% carbon monoxide (CO). The CO concentration of the treated reformate is lower than 20 ppm in the steady-state and is under 30 ppm at 65% load change. Hydrogen loss in the steady-state is about 1.5% and the pressure drop across the reactor is 4 psi. Start-up characteristics of the 10-kWe PROX system are also investigated. It takes 3 min to reduce the CO concentration to below 20 ppm. Several controllable factors are found to shorten the start-up time.

Predicting the Onset of a Low-Frequency, Limit-Cycle Type of Oscillatory Flow Instability in Multitube Condensing Flow Systems

A means was developed for extending the predictive capability of the Equivalent Single-Tube Model (ESTM) to accurately predict the onset of a self-sustained oscillatory flow instability for a multitube condensing flow system. The model includes the effects of compressibility, subcooled liquid inertia, and thermal and flow distribution asymmetry. Previously, liquid inertia, a necessary mechanism for the instability, had not been modeled for a multitube system. Extensive experimental data was obtained for a two-tube system that verifies not only the predictive capability of the ESTM, but also its accuracy and its wide range of applicability.

Modeling the Characteristics of Thermally Governed Transient Flow Surges in Multitube Two-Phase Condensing Flow Systems With Compressibility and Thermal and Flow Distribution Asymmetry

In a tube-type condenser involving complete condensation, small changes in the inlet vapor flow rate momentarily cause very large transient surges in the outlet mass flow rate. An Equivalent Single-Tube Model (ESTM), based on the System Mean Void Fraction Model, is developed that predicts these transient flow surges for a multitube system; including the effects of compressibility as well as thermal and flow distribution asymmetry. The model is verified theoretical and experimentally. From a design perspective, the significant value of the ESTM is that it includes the primary physical mechanisms involved in such complex flow transients, yet is simple enough to be solved on typical “spreadsheet” software.

Modeling the Thermally Governed Transient Flow Surges in Multitube Condensing Flow Systems With Thermal and Flow Distribution Asymmetry

In a tube-type condenser involving complete condensation, small changes in the inlet vapor flow rate momentarily cause very large transient surges in the outlet liquid flow rate. An equivalent single-tube model is proposed that predicts these transient flow surges for a multitube system. The model, based upon a system mean void fraction model developed earlier, includes the effects of thermal and flow distribution asymmetry associated with each individual condenser tube in the multitube system. Theoretical and experimental verification for a two-tube system is presented.