Step Change In Temperature Research Articles

Effect of temperature step-changes and ramps on physiological and psychological responses of occupants in office buildings in summer

Thermal conditions with step changes and ramps in air temperature (Ta) are universal in buildings. However, a comprehensive understanding of the influences of these dynamic thermal factors on thermal comfort remains elusive. This study investigated the physiological and psychological responses 16 healthy college students, including thermal sensation vote (TSV), thermal pleasure (TP), positive and negative affect schedule (PANAS), skin temperature, blood pressure, wrist pulse, and electroencephalography (EEG). The experimental procedure included step-changes from an office room (26 °C) to a meeting room under five different Ta (22/24/26/28/30 °C) and subsequent ramping (down/up/cycles) back to 26 °C. After these different warm/cool thermal experiences, TSVs (−0.2 to 0.2) were insignificantly different and TPs (0.2–1.6) were significantly different (P = 0.006) at neutral condition of 26 °C. Specifically, TPs under neutral conditions were higher after a warmer thermal experience than after a cooler thermal experience. Under the same Ta, TSVs were significantly lower (P = 0.031) by approximately 0.5, when Ta ramping down compared to ramping up and stable warm conditions. The cheek temperature showed stronger correlations with TP and PANAS scores than with other body parts. The blood pressure was weakly correlated with TSV but exhibited a strong relationship with TP. Moreover, the TP and positive emotions exhibited strong negative correlations with wrist pulses and EEG at Theta (4–8 Hz). These findings offer insights into refining the prevailing thermal comfort models when considering dynamic thermal factors in shaping thermal perceptions.

Dynamic behaviour and performance evaluation of a biomass-fired organic Rankine cycle combined heat and power (ORC-CHP) system under different control strategies

The dynamic behaviour and performance evaluation of a biomass-fired organic Rankine cycle combined heat and power (ORC-CHP) system under two different control strategies are investigated in this study. The dynamic model is established, while the dynamic characteristics of key operating parameters (superheat degree, evaporating pressure and mass flow rate) with the step change of heat source temperature are examined. The effects of controlled superheat degree and controlled expander shaft work on dynamic behaviours of thermodynamic, economic and environmental performance are discussed and compared under different control methods (feedforward active control, PID passive control and combined control). Results indicate that the superheat degree is significantly influenced by the heat source temperature under the without control strategy, with the variation reaching 8.4 K. The combined control method exhibits the best control performance with overshoot and control response times of 1.2 K and 12.3 s, respectively. Compared to without control, the expander shaft work and ORC thermal efficiency of the control strategy of superheat degree increase by 3.2 kW and 0.8 %, while that of the control strategy of expander shaft work increase by 1.0 kW and 0.3 %, respectively. The control strategy of superheat degree presents a significant improvement in the economic and environmental performance of the biomass-fired ORC-CHP system. This study is expected to provide better guidance for the practical operation and control of biomass-fired ORC-CHP systems.

Symmetric prism characterisation test to infer heat generation and heat conduction properties of concrete

Performance-based concrete design can accelerate the acceptance of emerging lower carbon concretes to reduce the environmental impact. A semi-adiabatic symmetric concrete prism experimental and analytical approach is proposed as a pathway for concrete thermal characterisation. The prism design promotes 1-D heat flow conditions using a single insulated specimen. The prism dimensions (700 mm ×100 mm ×100 mm) are rationalised to suit usage as a characterisation specimen and mix compositions that replicate those of in-situ concretes can be assessed. The experimental spatial and temporal thermal profiles of a CEM I mix and a high GGBS replacement mix were measured during hydration and through a step change in temperature in the hardened state. Thermocouples and a distributed fibre optic sensor along the central axis of the symmetric prism were used for this purpose. Multi-physics modelling provided a platform to address thermal measurement data uncertainty, extract fundamental concrete thermal parameters and gain insight into the heat generation and heat flow physics in the concrete prism specimens. The complete heat generation and maturity time histories were accurately modelled and ‘concrete thermal footprints’ derived.

Physiological and perceptual responses to temperature step changes between cold and hot environments

Objectives. This study explores the effects of temperature steps on thermal responses to understand abrupt temperature shifts faced by heat-exposed workers during winter. Methods. Three temperature step changes with three phases (S20: 20–40–20 °C, S30: 10–40–10 °C, S40: 0–40–0 °C) were conducted. Phase 1 took 30 min, phase 2 took 60 min and phase 3 took 40 min. Eleven participants remained sedentary throughout the experiment, and physiological responses, thermal perception and self-reported health symptoms were recorded. Results. In temperature up steps, steady skin temperature and sweating onset were delayed, and heart rate dropped by 10 bpm from S20 to S40. In temperature down steps to cold conditions, individuals transitioned from thermal comfort to discomfort and eventually cold strain. Blood pressure increased in temperature down steps, correlating with temperature step magnitudes. Thermal responses to temperature steps of equal magnitude but opposite directions were asymmetries, which weakened as step magnitude increased. Thermal perceptions responded faster than physiological changes after temperature steps, while self-reported health symptoms lagged behind physiological responses. Conclusions. These findings contribute to expanding basic data to understand the effects of temperature step magnitude and direction.

A thermoregulation model based on the physical and physiological characteristics of Chinese elderly

Given the increasing aging population and rising living standards in China, developing an accurate and straightforward thermoregulation model for the elderly has become increasingly essential. To address this need, an existing one-segment four-node thermoregulation model for the young was selected as the base model. This study developed the base model considering age-related physical and physiological changes to predict mean skin temperatures of the elderly. Measured data for model optimization were collected from 24 representative healthy Chinese elderly individuals (average age: 67 years). The subjects underwent temperature step changes between neutral and warm conditions with a temperature range of 25–34 °C. The model's demographic representation was first validated by comparing the subjects' physical characteristics with Chinese census data. Secondly, sensitivity analysis was performed to investigate the influences of passive system parameters on skin and core temperatures, and adjustments were implemented using measurement or literature data specific to the Chinese elderly. Thirdly, the active system was modified by resetting the body temperature set points. The active parameters to control thermoregulation activities were further optimized using the TPE (Tree-structured Parzen Estimator) hyperparameter tuning method. The model's accuracy was further verified using independent experimental data for a temperature range of 18–34 °C for Chinese elderly. By comprehensively considering age-induced thermal response changes, the proposed model has potential applications in designing and optimizing thermal management systems in buildings, as well as informing energy-efficient strategies tailored to the specific needs of the Chinese elderly population.

Dual effect of activity levels and temperature step-change on human comfort and recovery time: Simulated short-term commute in winter

Individuals living in a location with access to public transport and a short distance from the workplace tend to walk for short commuting. Human activity levels are affected by the rate of walking while commuting, which affects thermal comfort and response. To investigate the human thermal response in commuting, a climate chamber experiment was conducted to simulate outdoor commuting (13 °C) and indoor rest recovery (24 °C) in winter. Standing and walking at 4 and 6 km/h were used to simulate temporary rest, walking, and running, respectively. Twenty healthy participants were asked to simulate commuting (13 °C) with standing/low, 4 km/h/medium, and 6 km/h/high activity levels for 8 min and sitting (24 °C) for 32 min. The physiological parameters and subjective perceptions of the subjects were recorded. The results showed that there were differences in the sensitivity of the parameters to changes in activity levels and temperature steps. Thermal sensation and mean skin temperature responded considerably to the dual effect of activity levels and temperature up-step; however, blood pressure was exclusively sensitive to temperature step-change. The elevated activity level of 1 met may increase the recovery time by 5–10 min. Functional paradigms for psychological and physiological recovery times in a neutral environment were developed, which could provide a potential solution for improving thermal satisfaction and the assessment of recovery times. The results of this study will guide improvement strategies for physiological recovery and thermal comfort after commuting.

Developing thermal prediction models for the elderly under temperature step changes

Daily transitions between indoor and outdoor environments involving temperature step changes have been extensively studied in young people, but little attention has been given to age-related differences and the development of thermal prediction models specifically for the elderly. To address this knowledge gap, a study was conducted in a climate chamber using warm-neutral-warm experimental sessions. Twenty-four elderly participants were selected from volunteers who met the health and body shape requirements. Thermal sensation vote (TSV) and mean skin temperature (MST) responses of the elderly were collected and analyzed. The results demonstrate that temperature step changes (3–9 °C) and neutral thermal experiences significantly influence the thermal responses of the elderly. Compared to the young, the elderly exhibit lower TSV, lower MST, lower skin heat loss, and different associations between TSV and skin heat loss. Leveraging these age-related differences, an MST prediction model was established for the elderly. The predicted MST can also serve as an input for the TSV prediction model established in this study. The TSV prediction model is based on skin heat loss and applicable to both transient and steady-state conditions. This study introduces a new approach for predicting the thermal responses of the elderly and enhances understanding of age-related differences in thermal responses.

Analytical Solution of Transient Heat Conduction through a Hollow Cylindrical Thermal Insulation Material of a Temperature Dependant Thermal Conductivity

The one-dimensional, cylindrical coordinate, non-linear partial differential equation of transient heat conduction through a hollow cylindrical thermal insulation material of a thermal conductivity temperature dependent property proposed by an available empirical function is solved analytically using Kirchhoff’s transformation. It is assumed that this insulating material is initially at a uniform temperature. Then, it is suddenly subjected at its inner radius with a step change in temperature. Four thermal insulation materials were selected. An identical analytical solution was achieved when comparing the results of temperature distribution with available analytical solution for the same four case studies that assume a constant thermal conductivity. It is found that the characteristics of the thermal insulation material and the pressure value between its particles have a major effect on the rate of heat transfer and temperature profile.

Determination of Thermal Time Constant of a Pt100 Temperature Sensor in Still Oil Using the Time Derivative Method

AbstractAn alternative method was developed to determine the thermal time constant of a relatively cumbersome temperature sensor in still oil. A relationship between the take‐off gradient, initial temperature, thermal time constant, and final steady‐state temperature was deduced. An experiment was then performed using a Pt100 temperature sensor to test the veracity of the relationship. The Pt100 temperature sensor was subjected to five different step changes in temperature from an initial temperature of 60 °C. This resulted in five exponential curves with five different take‐off gradients. The linearization method was used to determine the thermal time constant as a control experiment. The new method was then applied to the same data set. It agrees very well with the linearization method within the limits of experimental error.

Heat and mass transfer analysis of constitutive model with autocatalytic chemical reaction within the Jeffery–Hamel flow perspective

This article examines the heat and mass transfer capabilities of a constitutive model in a thermally evolving steady laminar Jeffery–Hamel flow through a convergent-plate channel, including streamwise conduction with step changes in uniform wall temperature. A Jeffery–Hamel problem with a simple shear flow is used to undertake a comparative computational analysis of the thermal behavior of a viscoelastic fluid subjected to autocatalytic processes. The flow is tracked in a purely radial orientation with the deployment of coupled stresses in momentum conservation. The computational solutions for the flow, temperature and concentration distribution, and heat and mass transfer coefficient of a viscoelastic fluid obeying the complex Oldroyd-B constitutive equation in laminar converging channel flows are established. The analysis of the impacts of the thermal radiation, the heat source, and the chemical reaction as an autocatalytic process is included in the model, which is valid for fully developed thermal and hydrodynamic flow conditions with a constant heat and mass flux imposed at the wall. In the diverging part of the channel, where vortex compression is the predominant flow topology, there exist patches of local flow compression. On the flow field, the modified relaxation and retardation parameters show an opposing behavior. An Oldroyd-B fluid exhibits higher interactions with nearby vortices in the divergent channel, allowing a complex flow structure. The viscoelastic characteristics are anticipated to change the homogeneous–heterogeneous reaction transport processes, offering tremendous potential for applications in associated sectors. The deceleration flow in the diverging channel and the acceleration flow in the converging channel augment the average Nusselt numbers.

Analytical Solution of Transient Heat Conduction through a Hollow Spherical Thermal Insulation Material of a Temperature Dependant Thermal Conductivity

The one-dimensional, spherical coordinate, non-linear partial differential equation of transient heat conduction through a hollow spherical thermal insulation material of a thermal conductivity temperature dependent property proposed by an available empirical function is solved analytically using Kirchhoff’s transformation. It is assumed that this insulating material is initially at a uniform temperature. Then, it is suddenly subjected at its inner radius with a step change in temperature. Four thermal insulation materials were selected. An identical analytical solution was achieved when comparing the results of temperature distribution with available analytical solution for the same four case studies that assume a constant thermal conductivity. It is found that the characteristics of the thermal insulation material and the pressure value between its particles have a major effect on the rate of heat transfer and temperature profile.

A Novel Hybrid MPPT Approach for Solar PV Systems Using Particle-Swarm-Optimization-Trained Machine Learning and Flying Squirrel Search Optimization

In this paper, a novel hybrid Maximum Power Point Tracking (MPPT) algorithm using Particle-Swarm-Optimization-trained machine learning and Flying Squirrel Search Optimization (PSO_ML-FSSO) has been proposed to obtain the optimal efficiency for solar PV systems. The proposed algorithm was compared with other well-known methods viz. Perturb & Observer (P&O), Incremental Conductance (INC), Particle Swarm Optimization (PSO), Cuckoo Search Optimization (CSO), Flower Pollen Algorithm (FPA), Gray Wolf Optimization (GWO), Neural-Network-trained Machine Learning (NN_ML), Genetic Algorithm (GA), and PSO-trained Machine Learning. The proposed algorithm was modelled in the MATLAB/Simulink environment under different operating conditions, for example, with step changes in temperature, solar irradiance, and partial shading. The proposed algorithm improved the efficiency up to 0.72% and reduced the settling time up to 76.4%. The findings of the research highlight that PSO_ML-FSSO is a potential approach that outperforms all other well-known algorithms tested herein for solar PV systems.

A robust MPPT approach based on first-order sliding mode for triple-junction photovoltaic power system supplying electric vehicle

This paper studies the problem of the energy system optimization for photovoltaic (PV) generator supplying the electric vehicle. The used photovoltaic generator was conceived around a high-efficiency triple-junction solar cell InGaP/InGaAs/Ge. To adopt the photovoltaic generator to the load and to track the maximum power points, a DC–DC converter feeding the electric vehicle is used. Due to the nonlinearity of the PV generator’s electrical characteristics on the one hand and meteorological conditions variations and load changes, on the other hand, robust control approach is used. It is based on the sliding mode control (SMC) technique. The maximum power point tracking (MPPT) approach for a PV system is well detailed. Moreover, a stability analysis of this approach is carried out. The PV system used in the comparative study is made up of a resistive load, a boost converter, and a high efficiency PV generator. Standard test conditions, a step change in solar radiation with a constant temperature, a step change in temperature with a constant solar radiation, a step change in load and solar radiation with a constant temperature, simultaneous step changes in load, temperature, and solar radiation, and partial shading conditions are the seven case studies that are investigated. They have the aim of evaluating the effectiveness and advantages of the SMC algorithm in comparison to those of perturb and observe (P and O) and three meta-heuristic algorithms. The latters are the particle swarm optimization (PSO), flower pollination (FPA) and the cuckoo search algorithm (CSA). Obtained results show that the SMC algorithm has the ability to sufficiently and rapidly track the maximum power points in the aforementioned cases. As a result, it is used in electric vehicles. The traction part of the electric vehicle is built around a three-phase permanent magnet motor (PMSM). The control strategy is based on indirect, field-oriented control. The conceived system and the control algorithms are all implemented in Matlab/Simulink. Significant results for a wide range of speeds, including the two operating modes under step changes in environmental conditions and under strong partial shading conditions, are presented and discussed.

Jeffreys heat conduction in coupled semispaces subjected to interfacial heating

A Jeffreys heat conduction problem for coupled semispaces subjected to the action of an interfacial heat source was defined. An analytical solution of the problem was derived for a polynomial specific power of the heat source using the Laplace transform approach. The asymptotic and parametric analysis was performed for different ratios of thermal conductivities K1,2, thermal diffusivities k1,2, thermal relaxation times τ1,2 and coefficients α1,2 indicating the relative contribution of Fourier heat conduction. It was found that Jeffreys heat conduction results in continuous variation of the contact temperature, whilst its particular case — Cattaneo heat conduction — is accompanied by a step change of the contact temperature at the initial time. Another finding is that the initial heat partition occurs due to the ratio of K1α1/k1 and K2α2/k2 under Jeffreys heat conduction and due to the different ratio of K1/k1τ1 and K2/k2τ2 under Cattaneo heat conduction. The solution applicability was demonstrated on the simulation example of ultrashort laser pulse welding. The type of heat conduction was revealed to have qualitative and quantitative impacts on the contact temperature and heat fluxes.

Association between thermal response and endogenous dopamine: Step-change environments in winter

Temperature step change is the typical transient thermal environment. The purpose of this study was to explore the association of subjective and objective parameters in a step-change environment, including thermal sensation vote (TSV), thermal comfort vote (TCV), mean skin temperature (MST) and endogenous dopamine (DA). Three temperature step changes defined as I3 (15 °C–18 °C to 15 °C), I9 (15 °C–24 °C to 15 °C) and I15 (15 °C–30 °C to 15 °C) were designed for this experiment. Eight male and eight female healthy subjects who participated in the experiment reported thermal perception (TSV and TCV). Skin temperatures of six body parts and DA were measured. Results show that the inverted U-shaped in TSV and TCV was deviated by seasonal factors of the experiment. The deviation direction of TSV in winter was to the warm sensation side, which was opposite to the inherent cold and hot impression of people in winter and summer. The association between dimensionless dopamine (DA*), TSV and MST were described as follows: DA* was the U-shaped change with exposure times when MST was not greater than 31 °C, and TSV was at −2 and −1, and DA* increased with exposure times when MST was greater than 31 °C, and TSV was at 0, 1 and 2. The changes in the body heat storage and autonomous thermal regulation under temperature step changes may potentially be related to the concentration of DA. The human state on thermal nonequilibrium and stronger thermal regulation would correspond to a higher concentration of DA. This work is conducive to exploring the human regulation mechanism in a transient environment.

Graded Longitudinal Fins Having Spatially Varying Temperature-Dependent Thermophysical Properties

This paper reports transient responses of graded longitudinal fins subject to step change in base temperature and base heat flux wherein the graded fin materials are theorized to have spatial- and temperature-dependent thermal conductivity. Microstructure variations in graded materials (GMs) are addressed by axially varying the thermal conductivity; because GMs are potentially high-temperature application materials, consequently, thermal conductivity and heat generation are, respectively, assumed as polynomial and linear functions of temperature. Additionally, most of the applicable pragmatic fluid regimes are accounted for using the power law convection coefficient. The numerical solution of a typical nonlinear governing differential equation is obtained by using a particle tracking-based method called the lattice Boltzmann method (LBM). The LBM is a mesoscopic-based simulation method centered around the principles of kinetic theory and statistical mechanics. The LBM formulation accompanied with the in-house MATLAB code of the aforesaid problem with varying parameters is reported; also, it is validated with a previously available solution. The foregoing analysis is carried out to enhance the performance of a fin by using the superior thermomechanical property of graded materials. Furthermore, the inclusion of temperature-dependent thermophysical properties and heat generation will provide more accurate design data. The reported graph reveals that, even though a linear GM fin tip possesses thermal conductivity that is 25% less in magnitude in comparison to the Type-II homogeneous material (HM-2), the GM fin always yields a higher fin tip temperature because of grading. In addition, the tip temperature deficits between GMs and HM-2 proportionally increase from 0.4 to 2.1% for values of increasing from 0.1 to 2.0, respectively, for step changes in temperature; whereas in the case of the step change base flux, the deficits increase from 8.72 to 12.1% for values of decreasing from 3.0 to 1.0, respectively.

Study on the influence of hypobaric hypoxia on human thermal comfort under step change thermal environments

Given the meteorological conditions on the Tibetan plateau such as low temperatures and large diurnal range, people moving in and out of buildings are inevitably exposed to the transient, step change thermal environments. Significant temperature changes are closely related to humans' thermoregulatory processes, and the hypoxic environment may affect the thermoregulation process of sojourners in Tibet, which makes it difficult for people to adapt to temperature changes in time and increases the risk of disease. Nevertheless, most experimental studies on temperature step change were carried out in plain. Only a few experimental studies focused on the effect of temperature step change on human thermal comfort under low temperature and hypoxic environment. In order to understand the impact of this condition, the same temperature step change experiment was conducted in Xi'an and Lhasa, respectively. The results showed that the hypoxic environment of the plateau could cause decreasing of sojourners’ thermal sensation and skin temperature in Tibet under low temperature environment, and prolong the time needed for skin to rewarm. The results of this study can help provide a better thermal environment in buildings to ensure the health and thermal comfort of Tibetan sojourners.

Transient Flow Uniformity Evolution in Realistic Exhaust Gas Aftertreatment Systems Using 3D-CFD

To precisely control a vehicle powertrain to minimize emissions, accurate and detailed models are needed to capture the spatio-temporal variability of the variables of interest. The aim of this work is to analyze flow and temperature fields in a geometrically realistic — and thus complex — exhaust gas aftertreatment system under transient conditions. The spatio-temporal response of these fields to upstream step changes is predicted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) kappa - omega simulations where the catalytic converter is described as a porous medium. A catalytic converter geometry with a 90^{circ }-bend and a partially dead volume is used to demonstrate the effects of time-resolved flow maldistribution on the profiles of velocity and temperature. Two sets of transient simulations in terms of step changes in velocity and temperature are performed. Uniformity indices are used to characterize the distribution and variability of the different catalyst channels under transient conditions. The evolution of the uniformity indices as functions of time and axial distance into the catalyst are calculated at different cross-sectional planes. The results show that the evolution of the temperature uniformity is rate controlling, continuously modulating the otherwise much faster flow uniformity response via the fluid properties. The temperature uniformity time scale is determined by the balance of flow, thermal inertia, and the heat losses from the system. The interplay between pressure drop and heat losses governs the transition to the new steady state in uniformity. These types of transient simulations and analyses can contribute essential information when developing reduced-order engineering models to represent the spatio-temporal variability in exhaust aftertreatment systems, in particular during rapid events such as cold start.

Hydrocarbon Products Occluded within Zeolite Micropores Impose Transport Barriers that Regulate Brønsted Acid-Catalyzed Propene Oligomerization.

Brønsted acid zeolites catalyze alkene oligomerization to heavier hydrocarbon products of varied size and branching. Propene dimerization rates decrease monotonically with increasing crystallite size for MFI zeolites synthesized with fixed H+-site density, revealing the strong influence of intrazeolite transport limitations on measured rates, which has gone unrecognized in previous studies. Transient changes in dimerization rates upon step-changes in reactant pressure (150-470 kPa C3H6) or temperature (483-523 K) reveal that intrazeolite diffusion limitations become more severe under reaction conditions that favor the formation of heavier products. Together with effectiveness factor formalisms, these data reveal that product and reactant diffusion, and consequently oligomerization rates and selectivity, are governed by the composition of hydrocarbon products that accumulate within zeolitic micropores during alkene oligomerization. This occluded organic phase strongly influences rates and selectivities of alkene oligomerization on medium-pore zeolites (MFI, MEL, TON). Recognizing the coupled influences of kinetic factors and intrazeolite transport limitations imposed by occluded reaction products provides opportunities to competently tailor rates and selectivity in alkene oligomerization and other molecular chain-growth reactions through judicious selection of zeolite topology and reaction conditions.

Experimental evaluation of thermal comfort, SBS symptoms and physiological responses in a radiant ceiling cooling environment under temperature step-changes

People usually experience transient thermal environments when entering or leaving a conditioned indoor environment. This has been previously explored but there is little knowledge on the impact of temperature step-changes on thermal comfort in a radiantly cooled environment. We aim to investigate human comfort and underlying physiological mechanism in such conditions. We assessed thermal comfort, sick building syndromes (SBS) symptoms, and physiological responses. Twenty healthy participants were exposed to three temperature step-change conditions with three outdoor air temperatures (29 °C, 33 °C and 36 °C) and one indoor air temperature of 26 °C. Subjective evaluation was collected through a questionnaire. Blood oxygen saturation (SpO2), skin temperature, and electrocardiograph (ECG) were measured. As expected, the overall thermal sensation, comfort, acceptability, preference, and subjective air freshness changed significantly before and after temperature step-changes. Perceived sweat and chest tightness were also affected by the temperature step-changes. Skin temperature, heart rate, time-domain, and nonlinear heart rate variability were affected significantly under temperature step-changes. We observed the overshoot phenomenon with thermal sensation and subjective air freshness under temperature down-step. Thermal sensation had a faster stabilization time than the measured physiological parameters (i.e., skin temperature, heart rate and heart rate variability) under temperature step-changes. The stabilization time before starting a thermal comfort experiment should be at least 30 min. Thermal sensation and skin temperature had an asymmetry effect on temperature step-changes.