Reduction In Thermal Performance Research Articles

Influence of the outer skin on the flexural properties and thermal conductivity of densified Dendrocalamus asper bamboo

In this work, the effect of densification and the maintenance of the outer skin on the flexural properties and thermal conductivity of Dendrocalamus asper Backer ex K.Heyne bamboo was investigated. The results showed improvements in bending resistance for the densified samples compared to the un-densified bamboo. Densified samples with the outer skin showed the highest bending strength. Samples with the outer culm wall in compression offered the best results, with an average modulus of rupture (MOR) of around 348.9 MPa and modulus of elasticity (MOE) of 28.4 GPa. These values are around 40% and 86% (MOR), and 17% and 30% (MOE) higher compared to densified samples without the skin and un-densified samples, respectively. The fracture sections of specimens after bending tests were analyzed using Scanning Electron Microscopy (SEM) to visualize crack propagation within the outer skin and bamboo tissues. Maintaining the skin increases the thermal conductivity, and the densified bamboo with the skin has a thermal conductivity of around 0.23 W/m.K, 12.8% higher than the densified bamboo without skin and 22.5% higher than un-densified bamboo. Although a reduction in thermal performance was achieved, it was concluded that is reasonable to keep the bamboo skin, as it improves the overall mechanical resistance and reduces material loss during bamboo processing.

Numerical Simulation of Thermal Conductivity and Thermal Stress in Lightweight Refractory Concrete with Cenospheres

The main objective of this paper was to investigate the heat transfer of modified lightweight refractory concrete at the microscopic scale. In this work, such material was treated as a porous composite based on the compound of calcium aluminate cement and aluminosilicate cenospheres. The presence of air inclusions within the cenospheres was an essential factor in the reduction in thermal performance. Due to the intricacy of the subject investigated, our research employed numerical, theoretical, and experimental approaches. Scanning electron microscopy (SEM) imaging was performed to study the composite microstructure with a special focus on geometry, dimensions, and the distribution of cenospheres. Based on the experimental analysis, simplified geometrical models were generated to reproduce the main features of the composite matrix and cenospheres. A finite element framework was used to determine the effective thermal conductivity of such domains as well as the thermal stresses generated in the sample during the heat flow. A considerable difference in thermal properties was revealed by comparing the simulation results of the pure composite matrix and the samples, indicating a varying arrangement of cenosphere particles. The numerical results were complemented by a theoretical study that applied analytical models derived from the two-phase mixture theory-parallel and Landauer. A satisfactory agreement between numerical and theoretical results was achieved; however, the extension of both presented approaches is required.

Thermal Performance of Load-Bearing, Lightweight, Steel-Framed Partition Walls Using Thermal Break Strips: A Parametric Study

Thermal bridges are a very relevant issue for lightweight steel-framed (LSF) construction systems given the high thermal conductivity of steel, which can negatively compromise their thermal behaviour, reduce their durability, and decrease the building energy efficiency. Several thermal bridge mitigation strategies exist, including the attachment of thermal break strips (TBS) to the steel studs’ flanges as one of the most widely employed techniques. In this research, the relevance of TBS to the thermal performance improvement of load-bearing LSF partition walls was assessed by performing a parametric study, making use of a validated 2D numerical model. A sensitivity analysis was performed for five different key parameters, and their importance was evaluated. The assessed parameters included the number of TBS and their thickness, width, and thermal conductivity, as well as the vertical steel stud spacing. We found that these parameters were all relevant. Moreover, regardless of the TBS thermal conductivity, it is always worth increasing their thickness. However, the increase in the TBS width does not always lead to increased thermal resistance; a thermal performance reduction was noted when increasing the width of the TBS at higher thermal conductivities. Therefore, it was concluded that it is more efficient to increase TBS thickness than their width.

Analysis of silica fouling on nonwetting surfaces.

Ground water sources used as coolant fluids in a variety of thermal systems such as heat exchangers and power plant condensers contain silica particles that accrete on heat transfer surfaces over time leading to reduction in thermal performance, a problem that is particularly exacerbated with temperature. Nonwetting superhydrophobic, lubricant-infused, and a new class of solid-infused surfaces introduced in this work are candidates for fouling mitigation, by virtue of their water repellency, but little is known about fouling of silica on the surfaces, especially under dynamic flow conditions and as a function of temperature. This article presents, for the first time, a systematic study of dynamic flow fouling of silica on nonwetting surfaces vis-à-vis conventional copper surface over a temperature range 20-50 °C. The mechanism of silica aggregate formation and its adherence to the different surfaces is elucidated by scanning electron microscope (SEM) imaging. Sigmoidal growth model is used to describe the time evolution of fouling thermal resistance and an Arrhenius model is presented for the temperature-dependent increase in the asymptotic fouling resistance on nonwetting and conventional surfaces alike. Lubricant-infused and solid-infused surfaces are shown to reduce fouling thermal resistance by up to 25% and 13%, respectively, compared to a conventional surface, whereas superhydrophobic surfaces lose their non-wettability under flow conditions, leading to an adverse increase in the fouling resistance by up to 13%. Considering the possible lubricant depletion in lubricant-infused surfaces over prolonged exposure to a flowing fluid, solid-infused surfaces present a robust alternative.

Safety considerations for SPIcDER: Spacesuit integrated carbon nanotube dust ejection/removal system

This paper presents a multi-tier safe implementation of the Spacesuit Integrated Carbon Nanotube Dust Ejection/Removal (SPIcDER) system. SPIcDER mitigates dust contamination of spacesuits and flexible surfaces during surface exploration of Moon and Mars. SPIcDER has Carbon nanotube (CNT) fibers embedded within the spacesuit outerlayer. These are energized using high voltage, low power, Alternating Current (AC) signals (∼350–1000 V). Laboratory experiments show the time-varying electric fields produced by SPIcDER repel the dust.This system mitigates astronaut exposures to lunar dust, and protects spacesuits and flexible surfaces from dust abrasion, wear and tear, and reduction in thermal performance. The adherence of lunar dust, and its entry into the habitable volume of habitats/landers, which was a major problem during Apollo missions, is also minimized.In this study, we review the primary health and safety aspects of SPIcDER for astronauts operating in these spacesuits. Simulation using ANSYS Maxwell® and preliminary experimental investigations were performed to conduct safety analysis and provide recommendations for implementation of SPIcDER for lunar mission operations.

Wind-driven ventilation of Double Skin Façades with vertical openings: Effects of opening configurations

Insufficient airflow in the cavity of Double Skin façade (DSF) systems has resulted in the overheating risk of the cavity and the resulting reduction in thermal performance in cooling dominant climates. Previous research has focused more on buoyancy-driven natural ventilation, and there is a lack of research on utilising wind-driven ventilation in the cavity as a dominant natural force for the heat dispersion from the cavity. Additionally, geometrical features of the cavity and wind direction as influential factors on wind-induced airflow have not been addressed in DSF studies. This paper presents a detailed evaluation of the impact of opening configurations on the ventilation performance of an integrated tall building with DSF with respect to four wind orientations (0°, 30°, 60°, and 90°) to the facade. The evaluation is based on three ventilation performance indicators: (i) induced airflow rate, (ii) wind speed ratio, and (iii) airflow distribution across the cavity length. High-resolution 3D steady RANS CFD simulations of cavity ventilation were performed for a range of sizes of the DSF cavity. The CFD simulations were validated against wind-tunnel measurements. The results show that the ratio between the areas of the front and lateral openings is a critical factor for improving the ventilation in the cavity at the wind incident angle 0° < ϴ < 30°. As wind direction increases to 60° and 90°, cavity size tends to be of the highest importance. Employing three front openings on the external skin of DSF (M3) can increase the efficacy of the system in a broader range of wind direction and cavity size.

Optimization of Converging and Diverging Microchannel Heat Sink for Electronic Chip Cooling

This article presents a steady-state 3-D conjugate heat transfer and single-phase liquid flow simulation in the microchannel heat sink (MCHs). The 3-D governing equations with boundary conditions are solved for a constant heat flux of 200 W/cm2, applied at the bottom surface of MCHs. An array of microchannels with three different cross-sections—uniform, converging, and diverging—are investigated for a fixed chip dimension of 20-mm length, 4-mm width, and 0.15-mm substrate thickness. This article aims to present a comparative analysis of the hydraulic and thermal performance of MCHs with three different cross-sections. Simulations are performed for volume flow rate and microchannel depth range of 20–140 mL/min and 0.25–1.25 mm, respectively, with silicon microchannel using water as the working fluid. Numerical simulations are performed using COMSOL Multiphysics 5.3. Comparative analysis shows that for a wide range of volume flow rate, diverging channels MCHs shows superior thermohydraulic performance. Furthermore, it is observed that with an increase in depth or aspect ratio of the channel, there is a significant reduction in pumping power requirement with some marginal reduction in thermal performance. Furthermore, to calculate the optimized channel dimensions for minimum thermal resistance and pressure drop, a MATLAB code was written. Based on the results, the optimized channel dimensions are identified. The work presented and discussed in this article is relevant and beneficial for designing compact MCHs for electronic cooling applications.

Microstructure and blowing agent concentration analysis in accelerated aged polyurethane & polyisocyanurate insulation

Some closed cell foam insulation products show an increase in thermal conductivity at low temperatures. This reduction in thermal performance has been attributed to the diffusion of air and blowing agent through the foam and to the condensation of the blowing agent. Aging processes and polymer degradation further increase the thermal conductivity of foams. The initial cell structure plays a role in dictating the thermal performance and changes with foam aging which is rarely investigated. To understand the loss of thermal performance in closed cell foams, a microstructure and chemical characterization of pristine and aged samples was performed in this study. The aging behaviour was analyzed by SEM imaging and by measuring the blowing agent concentration in the foam. Changes in the polymer physical attributes were identified. This study also used gas chromatography and quantified changes in pentane concentration in aged polyisocyanurate foams. Results show that aged foams undergo change in their polymer appearance and cellular elongation. Gas chromatography quantified a decrease in blowing agent in the range of 11-85% for polyisocyanurate foams after aging.

Eco-friendly and Durable Antibacterial Cotton Fabrics Prepared with Polysulfopropylbetaine

A novel antibacterial agent polysulfopropylbetaine (PSPB) bearing carboxyl groups was synthesized and its application on cotton fabric to provide durable antibacterial property was also presented. The successful synthesis of PSPB and its immobilization onto the cotton fabric surface were verified by a series of tests including FTIR, 1H NMR, XPS and SEM. Viable cell counting method was employed to investigate antibacterial properties of the finished cotton fabrics. It was found that the cotton fabrics treated with PSPB were endowed with desirable antibacterial activity against both gram-negative bacteria Esherichia coli (E.coli, AATCC 6538) and gram-positive bacteria Staphylococcus aureus (S.aureus, AATCC 25922), with the bacterisotatic rates of 99.69 % and 99.95 %, respectively. Notably, the bacterial reduction rates still maintained over 90 % against both bacteria even after 50 consecutive laundering cycles. Moreover, tests concerning the hydrophilicity, air permeability, water vapor transmission, mechanical properties as well as thermal properties were carried out systematically. The experimental results indicated the hydrophilic performance, air permeability and moisture penetrability of the cotton fabrics finished with PSPB were improved greatly in spite of a slight reduction in thermal performance and little obvious influence on mechanical performance. The antibacterial cotton fabric has the potential to be applied in sportswear, underwear, household textiles, medical fields and much more.

Truncation effects in an evacuated compound parabolic and involute concentrator with experimental and analytical investigations

A two-stage line-axis solar concentrator composed of parabolic and involute reflectors with tubular absorber, have been designed and experimentally analyzed for thermal applications. The concentrator is covered by evacuated glass tube for low-heat loss configuration. Due to steep angle at the end of compound parabolic reflector, the concentrator tolerates to truncate a portion of the reflector with only slight reduction on the performance. Optimum truncation level is estimated by evaluating of ray acceptance, which is described as the ratio of aperture area of the concentrator to diameter of glass cover, concentration ratio, optical performance and thermal performance. The concentrator with truncation level of 50% shows preferable performance with only 1% reduction in thermal performance as well as significant reduction on material requirement. An experimental analysis was conducted for a 50% truncated concentrator to validate the ray tracing analysis and the thermal performance as a function of inlet temperature of water and the surface temperature of absorber. The experimental results show good agreement with the ray-tracing program written for theoretical evaluation. Consequently, the truncation of the concentrator provides requirement of less amount of optical component through significant reductions in size of reflector and glass cover with a small reduction in concentrator rate to achieve the economic viability and attraction for buildings.

An approach to fouling allowances in the design of compact heat exchangers

It is well known that the straight-forward application of fouling factors such as those recommended by TEMA is often inappropriate for compact exchangers. This is because in many applications they result in unrealistically high surface area increments, which in turn mean reduced throughflow velocities and increased fouling propensity, thereby counteracting their raison d'etre. It is argued in this paper that if a mean thermal conductivity of the deposit or fouling layer can be estimated for given process conditions, the application of a fouling factor R f in m 2 K/W implies a calculable deposit thickness. It is proposed that the acceptable deposit thickness, and hence R f, should be scaled in proportion to the hydraulic diameter to give the same proportional decrease in flow area, which in turn gives a calculable increase in pressure drop. Then provided that 1/ R f is in reasonable proportion to the clean overall heat transfer coefficient, the reduction in thermal performance and the increase in pressure drop will be acceptable. Values of reduced R f calculated in this way are in broad agreement with those recommended for both shell and tube exchangers and plate exchangers (PHEs). The approach puts an increased emphasis on the need for more direct data on deposit conductances, rather than derived values from R f values, although these can be used for first approximations.

Thermal Performance Reduction in Air-Cooled Heat Exchangers Due to Nonuniform Flow and Temperature Distributions

The effect of nonuniform (or maldistributed) inlet airflow and temperature on the thermal performance of cross-flow air-cooled heat exchangers, with both fluids unmixed, and taking into consideration differences in performance characteristics of individual tube rows, is addressed in this article. The latter may be due to different fin pitches or other geometric variations or row effects due to changes in air turbulence. Downstream tube rows in such a multirow tube bundle experience higher performance reductions than the tube row at the air inlet. A nonuniform air velocity distribution to a tube row leaves the row with a distorted temperature profile. This temperature nonuniformity increases as the air passes through subsequent tube rows, and causes the downstream rows to be progressively less effective. The present analysis is used to evaluate the performance of the individual tube rows and ultimately the entire bundle. Velocity distributions which have been measured on air-cooled heat exchanger models ar...

Studies on a high temperature regenerative heat exchanger for closed cycle MHD power generation. (Suppression of pressure loss)

Optimized design conditions of pebble bed regenerative heat exchangers are discussed from the standpoint of minimizing their pressure loss with a minimum reduction in thermal performance. Blow-down experiments for heating argon by hot air or by combustion gas were carried out to measure the pressure loss of argon in a pebble bed. Suitable equations for predicting the pressure loss were selected based on the measured results. The optimum values of pebble diameter and the mass flow rate of a working gas were determined by calculating pressure loss and thermal performance of pebble bed regenerative heat exchangers under cyclic continuous operation. The use of helium as a working gas in closed cycle MHD, instead of argon, proved to be advantageous from the viewpoint of both pressure loss and thermal performance.