Dynamic Temperature Response Research Articles

Decomposition thermokinetics and heat balance analysis of depressurized methane hydrate deposits under poor heat transfer conditions

Methane hydrate is an important form of natural gas resource found in submarine deposits, with exploitation efficiency mainly determined by in-situ hydrate decomposition. During the depressurization exploitation process of large-scale hydrate deposits, the heat transfer situation of far-wellbore deposits is highly likely to be worse than that of near-wellbore deposits; However, the thermodynamic and kinetic characteristics of hydrate decomposition under poor heat transfer conditions are still unclear, particularly lacking heat balance analysis theory. This study adopted an external temperature control technique in experiments to worsen the heat transfer situation and investigate its effects on the decomposition thermokinetics of depressurized methane hydrate deposits to 2.9 MPa. The stable decomposition rates of methane hydrates under different heat transfer situations were found to be linear with temperature difference, and decomposition stagnation was observed as heat transfer worsened. Based on these results, a novel heat balance model of depressurized deposits with hydrate decomposition was developed, which can accurately predict the dynamic temperature response of hydrate decomposition deposits with a deviation of no >8.9 %. In addition, the low hydrate decomposition rate under a worse heat transfer condition induced synergistic fluctuations in pressure and temperature, which can be improved with progressive warming. This study reveals the thermodynamic and kinetic behaviors of methane hydrate decomposition under poor heat transfer conditions for the first time and can help predict the spot exploitation characteristics of far-wellbore hydrate deposits.

A semi-analytical method based on the Green's function for the laser melting process of the metal materials

Thermal responses of the metal materials subjected to intense laser irradiation are complicated due to the phase change in the melting process. In order to simplify this problem, previous studies usually ignored the liquid phase or took the usage of finite element method, which limited the achievement of theoretical breakthroughs. In the present study, a one-dimensional heat transfer model with consideration of phase change was established to describe the melting process of the metal materials caused by a constant intense laser beam. The governing equations with three domains were constructed in the piecewise form: the liquid phase region (melted already), the solid phase region (unmelted yet) and the melting region. The limited discretization method was employed to describe the moving of the soli-liquid interface and the energy exchange. The piecewise governing equations in liquid and solid region were solved separately and analytically by means of the Green's function method, and the calculation result was verified by the finite element simulation. The results of the analytical model were in good agreement with those of the finite element simulation, and the maximal prediction errors of the temperature field prediction between the theoretical model and FE simulation is limited to 2.8 %. In addition, the influences of heat source intensity, material heat conductivity, latent heat on the melting process were discussed in detail. The analytical and simulation results reveal that the semi-analytical solution in piecewise for 1-D laser melting process can be successfully used to reflect the dynamic temperature response of the metal materials and the movement of the solid-liquid interface. This work is of guiding significance for the analytical solution of the multi-dimensional melting process of the metal materials under the fixed and continuous laser beams.

Detection of alpha heating in JET-ILW DT plasmas by a study of the electron temperature response to ICRH modulation

In the JET DTE2 campaign a new method was successfully tested to detect the heating of bulk electrons by α-particles, using the dynamic response of the electron temperature T e to the modulation of ion cyclotron resonance heating (ICRH). A fundamental deuterium (D) ICRH scheme was applied to a tritium-rich hybrid plasma with D-neutral beam injection (NBI). The modulation of the ion temperature T i and of the ICRH accelerated deuterons leads to modulated α-heating with a large delay with respect to other modulated electron heating terms. A significant phase delay of ∼40° is measured between central T e and T i, which can only be explained by α-particle heating. Integrated modelling using different models for ICRH absorption and ICRH/NBI interaction reproduces the effect qualitatively. Best agreement with experiment is obtained with the European Transport Solver/Heating and Current Drive workflow.

Experimental study on the dynamic response of voltage and temperature of an open-cathode air-cooled proton exchange membrane fuel cell

Air-cooled proton exchange membrane fuel cells (PEMFCs) are energy-converting devices suitable for various scenarios. In the present study, the dynamic response of voltage and temperature of an air-cooled PEMFC stack with 75 thermocouples is experimentally studied under different loading and fan-controlling strategies. It is found the larger the ramp increment or step amplitude, the greater the voltage overshoot/undershoot (reaching 5.09 V), the longer the response time (reaching 11.23 s), and the higher the voltage non-uniformity (reaching 20.20 %). Under the same current, the stack generally performs better in the unloading process than in the loading process. Too low (or too high) fan pulse width modulation (PWM) will lead to insufficient (or excessive) cooling capacity, resulting in stack overheating (or water flooding). For the stack studied, a relatively low PWM of 40 % at low current is more suitable, while under a medium or high current a PWM of 60 % leads to better cell performance.

Dynamic modeling and uncertainty quantification of district heating systems considering renewable energy access

The district heating system (DHS) is one of the essential carriers for coordinating renewable energy with fossil energy and achieving flexible energy consumption. Considering the uncertainty of both the renewable energy units' power and the consumers' aggregate loads impact the transport process of the DHS, the thermal characteristics of the DHS transportation and the node temperature response under the source-load uncertainty need to be quantified and analyzed. This paper proposed a method to calculate the nodes' temperature dynamic response of the DHS under multiple source-load uncertainty scenarios. The prediction and combination methods of supply and demand probability intervals of source and load nodes under varying credibility levels were investigated. Additionally, the effects of source-load uncertainty boundary conditions on the DHS transport process were analyzed. The temperature response of the nodes was calculated based on the established dynamic DHS model under multiple source-load uncertainty scenarios. A DHS in Beijing was selected for case validation and analysis, and it has 90 nodes and 109 pipes. The simulation results showed that the relative average error of node flow rate in the dynamic DHS model was 3.02%, and the average difference in node temperature was 1 °C. The probability distribution semi-analytical method and the Bayesian credible interval calculation method can accurately describe the uncertainty on both source and load sides. The models and algorithms proposed in this paper could effectively quantify the nodes' dynamic temperature response under the source-load uncertainty.

Dynamic discharging performance of a latent heat thermal energy storage system based on a PID controller

A PID controller is introduced into a latent heat thermal energy storage unit to compose a coupling system in order to control the discharging performance. Outlet temperature of the working fluid can be precisely regulated by means of its inlet velocity variation based on the PID controller. The theoretical model is built through combining heat transfer of the latent heat thermal energy storage unit with the PID controller. Experimental results are used to validate the proposed theoretical model. PID control analysis indicates that its parameters have obvious effects on performance of the coupling system. Kp of −0.02 m/(s·K), Ki of −0.15 m/(s2·K) and Kd of −0.001 m/K are further optimized according to dynamic response of the transient outlet water temperature. System parameter analysis exhibits that higher target temperature produces larger heat discharging rate, which then reduces the outlet water flow rate accordingly. The discharging rate and total discharging energy of the latent heat thermal energy storage unit decrease with augment of the target outlet temperature. Increase of PCM melting point, thermal conductivity and latent heat is favorable of elevating the working fluid velocity. The discharging rate is improved by the PCM melting point, thermal conductivity and latent heat. The latent heat thermal energy storage units can separately obtain maximum discharging rates of 257.691, 294.437 and 257.603 W at 200 min for PCM melting point of 327.15 K, PCM thermal conductivity of 0.8 W/(m·K) and PCM latent heat of 250 J/g. It is apparent that PCM melting point, thermal conductivity and latent heat are conducive to enhancing the total discharging energy. The largest discharging energy at 200 min is respectively determined as 1280.409, 1060.105 and 974.153 kJ. Temperature and phase change contours can also be substantially affected by the system parameters. In conclusion, the built coupling system is beneficial to efficiently regulate and control discharging performance of latent heat thermal energy storage units.

Measurement of the dynamic temperature response of electrocaloric effect in solid ferroelectric materials via thermoreflectance

ABSTRACT We propose a characterization method based on modulated thermoreflectance technique for measuring directly the electrocaloric (EC) effect. First, the EC response in BaTiO3-based multilayer capacitors (MLC) with Y5V specification was measured using thermoreflectance set-up. Further, the direct measurement method was implemented on P(VDF-TrFE) 55/45 thin films deposited on flexible PET substrate, where the frequency dependence of the EC response in these films was investigated. It was noted that the P(VDF-TrFE) EC temperature responses depend on the operation voltage frequency, thus we considered three frequency ranges using square voltage waveform. Three distinct types of EC temperature variation responses were observed depending on the waveform frequency range. In addition, the EC temperature variation of the thin film with a thickness of 1.4 µm was measured via thermoreflectance. The data reveal a large EC effect occurring at room temperature where a temperature change ΔT = 1.14°C was induced under a field of 82 MV/m.

Understanding the dynamic response of Durafet-based sensors: A case study from the Murderkill Estuary-Delaware Bay system (Delaware, USA)

The use of Durafet-based sensors has proliferated in recent years, but their performance in estuarine waters (salinity < 20) where rapid changes in temperature and salinity are frequently observed requires further scrutiny. Here, the responses of the Honeywell Durafet and its internal (pHINT) and external (pHEXT) reference electrodes integrated into a SeapHOx sensor at the confluence of the Murderkill Estuary and Delaware Bay (Delaware, USA) were assessed over extensive ranges of temperature (1.34–32.27°C), salinity (1.17–29.82), and rates of temperature (dT/dt; −1.46 to +1.53°C (0.5 h)−1) and salinity (dSalt/dt; −3.55 to +11.09 (0.5 h)−1) change. Empirical analyses indicated dynamic errors in the temperature and salinity responses of the internal and external reference electrodes, respectively, driven by tidal mixing were introduced into our pH time-series. These dynamic errors drove large anomalies between pHINT and pHEXT (denoted ΔpHINT−EXT) that reached >±0.8 pH in winter when the lowest temperatures and maximum tidal salinity variability occurred and >±0.15 pH in summer when the highest temperatures and minimum tidal salinity variability occurred. The ΔpHINT−EXT anomalies demonstrated a clear linear relationship with dSalt/dt thereby making dSalt/dt the strongest limiting factor of reference electrode response in our application. A dynamic sensor response correction for the external reference electrode (solid-state chloiride ion-selective electrode, Cl-ISE) was also developed and applied in the voltage domain. This correction reduced winter and summer ΔpHINT−EXT anomaly ranges by >40% and 68.7%, respectively. Summer anomalies were notably reduced to <±0.04 pH across all measurements. Further, this correction also removed the first-order salinity dependence of these anomalies. Consequently, dynamic errors in reference electrode response cannot be ignored and must be considered in future experimental designs. Further work to better understand the dynamic temperature and salinity responses of both reference electrodes is underway. Ultimately, we hope this work will stimulate further discussion around the role and treatment of large ΔpHINT−EXT anomalies as a part of future data quality control and data reporting as well as the dynamic errors in reference electrode response that drive them in the context of Sensor Best Practices.

Thermal Insulation Performance of Monolithic Silica Aerogel with Gas Permeation Effect at Pressure Gradients and Large Temperature Differences

ABSTRACT Silica aerogel is an excellent thermal insulator for high-speed aircraft, but there is little research on it in a high-temperature and complex-pressure environment. This research aims to evaluate the thermal insulation performance of silica aerogel monoliths with different porosities under large temperature differences and pressure gradients. We established an experimental system to measure and analyze the hot surface temperature response by fixing the heat flux and the cold surface temperature at transient pressure conditions. An unsteady-state heat transfer model considering gas flow is developed. The effective thermal conductivity of silica aerogels with 79.55 ~ 90.91% porosity is measured at different temperature differences between cold and hot surfaces (127 ~ 512 K), near-vacuum (<10 Pa), and transient pressure conditions. The results demonstrated that silica aerogel with 90.91% porosity showed the best thermal insulation performance when the temperature differences were over 500 K, while the aerogel with 79.55% porosity became the best when the temperature differences were less than 500 K. In addition, both the temperature and pressure difference affect the thermal insulation performance: the energy transport caused by gas flow affects the dynamic temperature response when gas permeability is of the order of 10−15 m2; the thermal insulation performance is improved by increasing gas permeability and pressure difference when gas flow and heat transfer directions are opposite.

Application and analysis of flip mechanism in the melting process of a triplex-tube latent heat energy storage unit

In order to improve the characteristics of uneven melting in the melting process of the horizontal latent heat energy storage system, the triplex-tube latent heat energy storage unit is taken as the research object, and the flip mechanism is applied to its melting process. Numerical simulation is used for the research, and the numerical model is verified by experimental data. The results show that under different dimensionless times, the melting performance of the unit can be significantly improved by a single flip. When the dimensionless time is 0.4576, it is found that the total melting time of the unit is reduced by 16.17 %, the average heat absorption rate is increased by 14.7 %, but the total heat energy absorption is reduced by 3.85 %. The results show that the addition of a flip can effectively shorten the melting time and increase the heat absorption rate, but it has a negative effect on the total heat absorption in one melting cycle. Moreover, through the comparison of dynamic flow rate, dynamic temperature response, and temperature interval, it is shown that the addition of flip effectively reduces the negative influence of the hard-to-melt zone on the melting performance of the unit during the melting process. The flip mechanism reduces the proportion of high-temperature phase change material in the melting process and makes the melting process more uniform.

Assessment on the melting performance of a phase change material based shell and tube thermal energy storage device containing leaf-shaped longitudinal fins

This work concerns the development of a leaf-shaped longitudinal fin for the melting performance enhancement in a shell and tube thermal energy storage device. A two-dimensional code with the consideration of natural convection is established to simulate the phase transition process in phase change material (PCM), and the reliability is validated through comparing of the numerical results with published experimental data. The dynamic temperature evolution and response of the leaf-shaped fin with enhanced performance is first analysed. The effects of the fin length, fin angle and fin material on the melting performance are then studied and discussed. Finally, the fin shape effect on PCM melting process is assessed by considering two different configurations of rectangular and triangular sub-branches under the same working conditions. The results show that the leaf-shaped fin with ever-increasing length of sub-branches offers extra thermal surface to contact the PCMs that located at the far side of the heating source, and hence achieves uniform heat transfer rate and accelerates the melting process in the device. For a fixed fin volume in the device, the fin length exhibits significant influence on the melting rate with the complete melting duration being shortened by 14.2 % when the value varies from 22 mm to 25 mm. Moreover, the performance enhancement induced by the leaf-shaped fin can be further strengthened by optimizing the fin sub-branch angle and shape. It is revealed that the fin with a branch angle of 75° achieves an acceleration of whole melting process by 12 % compared to the fin with branch angle of 90°. The total melting time in the fin with triangular shape of sub-branches can be reduced by 30.1 % in comparison with the fin with rectangular configuration of sub-branches.

Detection and recognition of toluene and butanone in mixture based on SnO2 sensor via dynamic transient and steady-state response analysis in jump heating voltage mode

The temperature modulation method is used to improve the selectivity of sensor, but there are still some deficiencies in the gas mixture recognition. The dynamic response law is insufficient understood because the dynamic response process itself is a complex change and there are some factors such as competition or suppression among multi-gases. This paper solves this problem by decomposing dynamic response and mapping static response. Based on the grain boundary barrier model, dynamic response is divided into dynamic temperature response and dynamic gas-sensing response. The dynamic temperature response is unaffected by the target gas and can be detected directly. Dynamic gas-sensing response cannot be detected directly but can be mapped through static responses. Therefore, this paper uses static response to grasp the dynamic response law to directionally optimize the experimental scheme. Dynamic transient response and dynamic steady-state response are analyzed one by one to discuss the role of dynamic response law in gas mixture recognition. High recognition rate is achieved in KNN recognition due to the selection of PCA dimensionality reduction results of the second dynamic transient response and dynamic steady-state response value. The study of the dynamic response law provides a new avenue for the recognition of gas mixture.

Dynamic Response of Surface Water Temperature in Urban Lakes under Different Climate Scenarios-A Case Study in Dianchi Lake, China.

Lake surface water temperature is a fundamental metabolic indicator of lake ecosystems that affects the exchange of material and energy in lake ecosystems. Estimating and predicting changes in lake surface water temperature is crucial to lake ecosystem research. This study selected Dianchi Lake, a typical urban lake in China, as the research area and used the Air2water model combined with the Mann-Kendall mutation statistical method to analyze the temporal and spatial variation in the surface water temperature of Dianchi Lake under three climate models. The research results show that, under the RCP 5-8.5 scenario model, the surface water temperature change rate for Dianchi Lake from 2015 to 2100 would be 0.28 ℃/10a, which was the largest change rate among the three selected scenarios. The rate of change during 2015–2100 would be 9.33 times higher than that during the historical period (1900–2014) (0.03 °C/10a). Against the background of Niulan River water diversion and rapid urbanization, the surface water temperature of Dianchi Lake experienced abrupt changes in 1992, 2016, 2017, and 2022. Against the background of urbanization, the impact of human activities on the surface water temperature of urban lakes will become greater.

Role of latent heat storage in thermal performance of an axially grooved heat pipe

Heat pipe coupled with phase change material (PCM) based latent heat storage (LHS) is a significant thermal management system that can efficiently control the intermittent high heat loads of electronic devices. Herein, an axially grooved heat pipe-LHS (AGHP&LHS) system has been proposed. The influence of LHS units and their location on dynamic temperature response of AGHP has been investigated both experimentally and theoretically. The effect of filling rates of phase change materials (PCMs) as well as cooling temperature on thermal response of the AGHP has been compared and analyzed. Results indicate that the LHS effectively stabilizes the heat source temperature under the intermittent heating process, enhances the thermostat performance and brings the temperature uniformity in AGHP. In addition, LHS units arranged at the evaporator section are optimal for AGHP under intermittent heat load conditions, reducing the heat source temperature by 27.4% and prolonging the melting time by 81.5%. As the heat load increases, the melting time of AGHP&LHS system becomes shorter, and the overall temperature uniformity is worsened. In particular, the melting time is prolonged by 34.8% for AGHP&LHS system with PCM filling rate of 89% than that of 44% filling rate.

Dynamic thermal analysis of startup process for minichannel evaporator

• Start-up regimes of minichannel evaporator are formulated by transfer function. • Evaluation criteria in control theory are proposed to describe start-up behaviors. • Startup performances of minichannel evaporator are quantitatively investigated. • Working fluid with small subcooling degree is beneficial for evaporator start-up. • Evaporator temperature overshoot can be alleviated via increasing heat load. The start-up performance of minichannel evaporator in a mechanically pumped two-phase loop (MPTL) is investigated in terms of the dynamic temperature response. A preliminary start-up regime diagram is identified, where temperature response of start-up process can be recognized as three modes, including the gradual start-up, once-rise overshoot start-up and twice-rise overshoot start-up. Theoretical analysis and model identification are conducted via Laplace-transformation and representative evaluation criterion is proposed to realize the quantitative characterization of specific start-up mode. Time constant and settling time act as the indicators to evaluate temperature response rate, while overshoot is utilized to assess the two-phase flow instability. The results indicate that: for gradual start-up mode, the time to reach quasi-steady state is smaller as heat transfer performance improves via increasing heat load, while temperature response rate is relatively insensitive to flow rate; for once-rise overshoot start-up mode, the two-phase flow instability can be alleviated via increasing the heat load, while the increment of flow rate could intensify the instability; for twice-rise overshoot start-up mode, the temperature overshoot is insensitive to heat load variation. Moreover, preheating the working fluid at evaporator inlet is beneficial to mitigate the instability, indicating that preheater/regenerator can optimize the robustness/stability of start-up.

Energy Saving Control of District Heating System Based on MATLAB

In order to offset the lag and delay in the heating process and solve the problem of excessive heating in the heating season, an energy-saving control method of district heating system based on MATLAB is proposed. Based on MATLAB/SIMULINK software, the simulation model of district heating system is established to study its dynamic characteristics. The short-term load forecasting method is introduced to guide the system to conduct centralized regulation and observe the dynamic response of indoor temperature and operating energy consumption. Compared with the constant water supply temperature and the outdoor temperature compensation method, the results show that the proposed method can reduce the indoor temperature fluctuation range to 20 ± 1°C, and the energy saving rate of the whole heating season can reach 14.5% compared with the conventional regulation method.

Influences of gas permeation with adsorption effect on the transient thermal insulation performance of silica aerogel at pressure and temperature differences

Silica aerogel is a thermal protective material for an outer space launch under aerodynamic heating, significant temperature difference, and pressure gradient conditions. This paper proposes a gas permeation model by considering the gas adsorption effect for silica aerogel under pressure differences. The gas (N2) adsorption behavior of silica aerogel is simulated by the grand canonical Monte Carlo method. This simulation provides the concentration of the adsorbate gas layer and the average thickness of adsorbate gas in the nanopore. An unsteady-state heat transfer model within silica aerogel, coupling with the acquired gas diffusion effect, heat conduction, and thermal radiation, is developed to study the thermal insulation performance. The gas permeation with the gas adsorption inside silica aerogel exerts a prominent influence on the dynamic temperature response of the hot surface. At the temperature of 77–90 K, the gas adsorption has a remarkable impact on the gas permeation within silica aerogel, which would finally affect the energy migration. The order of the coefficient changes from 10−14 to 10−10 m2. In contrast, the adsorption effect is negligible at the temperature of 298–1300 K. When the gas diffusion and heat transfer directions are opposite, gas diffusion will impede heat transfer, and thus the thermal insulation performance will be improved up to 88.94% and 25.65% at unsteady- and steady-state, respectively.

A Forward Feedback Control Scheme for a Solar Thermochemical Moving Bed Counter-Current Flow Reactor

Abstract Pelletized thermochemical energy storage media has a potential for long-duration energy storage. Production of solid-state energy storage media can be done within a cavity chemical reactor that captures concentrated solar radiation from a solar thermal field. The temperature stability of a solar reactor is directly influenced by the solar flux intercepted. This paper presents a low-order physical model to simulate the dynamic response of temperature inside a tubular plug-flow reactor prototype. Solid granular particles are fed to the reactor from the top whereas a counter-current flowing gas enters the reactor from the bottom. An in-house code was developed to model transient heat transfer of the reactor wall, gas, and moving particles. The model was preliminarily validated with packed beds for different temperature ranges and two gas flowrates. Dynamic response of the reactor temperature is simulated for different input power and gas/particle flowrates. The results show that the system response can be controlled efficiently by utilizing input power (solar flux) as a control parameter. A conventional proportional integral (PI) controller is designed to control the temperature inside the reactor and to maintain it during the solar flux intermittency. The controller parameters are tuned using the Ziegler–Nichols method to ensure optimal system response. The results show that the feedback control model is successful in tracking different reference reactor temperatures within a reasonable settling time of 30 min and eliminated overshoot. This study can be extended to include a hybrid reactor with a multi-input, multi-output variable system.

Comparison of solidification performance enhancement strategies for a triplex-tube thermal energy storage system

• The comprehensive application of fins and multistage inner tubes in TTES system is studied. • The difficult solidification zone of the system is found by dynamic temperature response. • The optimum position of multistage inner tubes and fins is explored by response surface method. • The ternary mixed molten salts of Li 2 CO 3 - K 2 CO 3 - Na 2 CO 3 ( 32 - 35 - 33 w t % ) is used as PCM. To enhance the solidification performance of phase change material in a triplex-tube thermal energy storage system, a two-dimensional model is established in this paper. The solidification performance of the system is studied by numerical simulation, and the method of adding multistage inner tubes and longitudinal fins is proposed to strengthen the solidification. Firstly, the size of eight fins in the initial model is studied to select the most suitable model, and the most difficult solidification area of the system is found through the study of dynamic temperature response at the position points. Then, multiple inner tubes and T-shaped fins are added to the area and a response surface method is used to optimize the design. The solidification performance of the system can be improved by 50.19% when the diameter of the multistage tube is 4 mm in the best position. The results show that enhancement of solidification property of phase change material by multistage inner tube is better than that by fin. The effects of physical parameters such as multistage inner tubes diameter, cold fluid temperature, and fin structure on solidification performance are also discussed. Finally, the effect of natural convection on the solidification process of phase change material is analyzed.

Melting performance assessments on a triplex-tube thermal energy storage system: Optimization based on response surface method with natural convection

The low thermal conductivity of phase change materials significantly affects the heat storage and release performance for a phase change energy storage system. To alleviate this issue, a novel triplex-tube latent heat thermal energy storage system is designed and the melting behavior of phase change materials is studied numerically. On the premise of a fixed total volume of phase change material, the multi-parameter optimization design of the system is carried out by the response surface method. The fluid-structure coupling function of each parameter is fitted. Compared with the original model for the triplex tube, the melting performance of the optimized model is improved by 23.87%. The internal temperature fluctuation is found by dynamic temperature response, and the mechanism is explained by dynamic flow rate study. The improvement of melting performance by natural convection after optimization is also studied. The influences of the physical parameters (fin length, initial temperature of phase change material, inner and outer tube wall temperature, and fin material) of the optimization model on the melting performance of the system are quantified, as well. The effect of fin length (30-35-40 mm) on melting performance is found to be less than 11.47% and the effect of thermal conductivity of fin on the melting process is not obvious.