Radiative Heat Transfer Processes Research Articles

Natural convection heat transfer in an eccentric annulus filled with hybrid nanofluid under the influence of thermal radiation and heat generation/absorption

This investigation aims to scrutinize the flow and heat transfer characteristics of a hybrid nanofluid composed of Ag-MgO within an eccentric annular space, where the inner circular wall is held at a constant elevated temperature and the outer circular wall is uniformly cooled. Natural convection in such eccentric annular enclosures, with thermal radiation and the effects of heat generation/absorption taken into consideration, plays a vital role in various engineering applications, including solar collectors, nuclear reactors, thermal storage systems, and heat exchangers. The resolution of the fundamental governing equations was executed employing the Gauss-Seidel approach, with the inclusion of a sub-relaxation process, all within the framework of the finite volume method. The impact of the Rayleigh number, radiation number, volume fraction as well as heat generation/absorption coefficient on thermal transfer, flow, and temperature distributions were investigated. It was observed that for low Rayleigh numbers, radiation influences enhance flow stability and improve the conductive heat transfer mode. Conversely, at high Rayleigh numbers, radiation intensifies convective heat exchange. The growth of the Rayleigh number diminishes the impact of heat generation and reinforces the radiation heat transfer process, while heat absorption exhibits the opposite effect. From Nr = 1 onwards, the average Nusselt number values are no longer influenced by an increase in nanoparticle volumetric fraction, and they are solely linked to the radiation number.

Minimum entropy generation paths for generalized radiative heat transfer processes with heat leakage

Heat transfer processes (HTPes) with generalized radiative heat transfer law (HTL) [q∝Δ(Tn)] and heat leakage (HL) are optimized. With net amount of heat transferred (NAHT) of working fluid at low-temperature side and given process period, optimal path, that is, optimal temperature configuration of reservoir (TCR), for entropy generation minimization (EGM) of HTP is derived. Universal result for optimal path is provided, and results for four special HTLs, that is, Newtonian, radiative, linear phenomenological, and mixed HTLs, are further provided. Optimal path with EGM is compared with traditional ones of constant heat flux rate (CHFR) and constant reservoir temperature (CRT). Effects of HL and NAHT on optimal results are analyzed. Results show that ratio of heat flux rates between reservoir and working fluid to (n1+1)/2 power of reservoir temperature for EGM of HTPes without HL is a constant, but the optimal relationship between temperatures of working fluid and reservoir with HL is significant different. The differences among TCRs with different heat transfer paths increase with increases of HL and NAHT. The path of CHFR is very close to that of EGM, and the two paths are superior to that of CRT.

Numerical simulation for the determination of the radiative properties of spherical packed bed porous media: A COMSOL Multiphysics Study

The determination of the radiative properties of porous media has become a critical issue in various industrial and engineering applications. The aim of this paper is to characterize the radiative heat transfer process through porous media, assumed to be spherical packed beds. A prediction model was developed using the software COMSOL Multiphysics to simulate the interaction of each of the three proposed structures with a plane-heating surface. The distribution of normalized fluxes was assessed allowing the computation of effective radiative properties, namely the transmissivity, reflectivity, and absorptivity for diffusely and specularly reflecting particles. The results show that the arrangement of the particles has a noticeable influence on the media properties. Two layers of the third model were enough to obtain an opaque surface. Correlations have been developed to allow effective reflectivity, transmissivity, and absorptivity coefficients to be easily and accurately defined as a function of emissivity in future models. The suitability of the proposed models was discussed through a comparative study of the results found using numerical simulations with analytical calculations, with a good agreement obtained.

A calculation method for the view factor between a wall and other feature surfaces under the influence of spherical canopy

Radiation heat transfer is one of the main modes of heat exchange for building exterior walls. When a tree is added to the site, the view factor (VF) between the exterior wall and surrounding surfaces, such as walls and floors, is affected, thus significantly impacting the radiation heat transfer of the exterior wall. In order to quantitatively analyze the impact of trees with spherical canopy on the radiation heat transfer process of exterior walls, a method for calculating the VF between the exterior wall and adjacent surfaces using the optical projection simulation method was proposed. The VF between the wall surface and the adjacent surface, as well as the visibility between the wall and ground surfaces when obstructed by the spherical canopy, can be approximately calculated. Finally, the VFs calculated are used to establish the radiation heat equations according to network diagram for radiation heat transfer. By comparing theoretical values with the measured values of irradiation intensity and radiosity intensity, it was found that the error rate of the optical projection simulation method does not exceed 3%. This method can be used to further study the effects of factors such as canopy's diameter, wall-canopy distance, and planting orientation on the disturbance of radiation heat transfer of building exterior walls.

Entropy and exergy analysis of coupled radiative heat transfer and heat conduction: A new thermodynamics approach

This study presents a new approach for evaluating the thermodynamic efficiency of a coupled radiative and conductive heat transfer process with an internal heat source. In this approach, the exergy balance equation is derived, and the exergy augmentation rate due to the internal heat source is found to be the sum of the exergy of the system output to the environment surroundings, the exergy stored in the system, and the exergy consumption rate due to the irreversibility of heat transfer in the system. The exergetic efficiency of the coupled heat transfer process is then obtained according to the radiative exergy balance equation. Based on the above theoretical derivation, we carry out a thermodynamic analysis of an example including five coupled heat conduction-radiation transfer systems with a uniform internal heat source and different boundary conditions. The variations in the numerical results are discussed according to the exergy balance principle, and the behaviours of the exergy components over time under transient conditions are analysed. Our numerical results show that increasing the area of the adiabatic boundary and reducing the heat loss of the system should be considered when designing heat-transfer systems (as the exergetic efficiency increases from 0.515 to 0.617). The larger the area of the adiabatic boundary of the system, the stronger the effect of increasing this area on the overall optimisation. When a system has two adiabatic boundaries, a symmetrical arrangement of the adiabatic boundaries is suggested, although this is much less effective than increasing the number of adiabatic boundaries.

A new WSGG radiation model of CO/CO2 mixed gas for solar-driven coal/biomass fuel gasification

Gasification driven by solar energy with CO2 is an ideal way of low-carbon resource utilization. However, there is a lack of research on the radiation heat transfer process which is important in gasification simulation. In this study, we developed a new the weighted-sum-of-gray-gases (WSGG) model to calculate the radiation heat transfer properties of CO and CO2 mixtures in solar-driven coal/biomass fuel gasification. Benchmarked against the statistical narrow-band model (SNB) of the EM2C laboratory, the WSGG model is suitable for the temperature range of 400–2500 K and the path length range of 0.001–60 m. This study also explored the effect of the CO/CO2 molar ratio on the overall emissivity of the mixture. Furthermore, the model introduces a pressure term into the emissivity calculation process and broadens the pressure range (1 bar, 5 bar, 45 bar). For the first time, the WSGG model is applied to the case where the H/C element ratio is 0, and the fluctuating temperature distribution case (1000 – 2000 K) is analyzed, which is suitable for coal/biomass fuel gasification. In addition, this study calculated the one-dimensional radiation transfer equation. The results show that the average radiation source term difference between the new WSGG and the benchmark SNB model is within 5 % in common solar gasification engineering conditions (5 bar, 5 m). Meanwhile, this study also clarified the effect of pressure on the radiation heat transfer with different temperatures.

An improved model for performance predicting and optimization of wearable thermoelectric generators with radiative cooling

Wearable thermoelectric generators (WTEGs) have shown great potential for harvesting low-grade body heat. However, inappropriate design of cold side heat sinks leads to unsatisfactory power generation efficiency. To overcome this, radiative cooling cold side has been introduced by some earlier researchers. Nonetheless, a reliable performance evaluation model lacks for WTEGs with radiative cooling at outdoors. In this paper, we propose a novel analytic model, which considers comprehensive thermoregulatory mechanisms and detailed radiative heat transfer process. The model demonstrates good accuracy with less than 9% error in a variety of conditions. Furthermore, the proposed dynamic model greatly outperforms the constant model and static model in terms of WTEG output performance prediction at outdoors, which can reduce the deviations by up to 196.4% and 71.6%, respectively. Among the three types of cold side structures (bare, finned, and radiative cooling), we found that the radiative cooling cold side performed best. Accordingly, the output performance of the WTEG with radiative cooling was further investigated, and the influences of climatic conditions and spectral properties of radiative cooling on WTEG were obtained. Different human body segments were also considered in outdoor simulations, with the shoulder giving the best performance. Finally, based on the proposed model, the thermal resistance improvement indicates that 220% output performance enhancement is possible, which could be beneficial for device design and utilization in many other applications.

Far‐ and Near‐Field Heat Transfer in Transdimensional Plasmonic Film Systems

AbstractA confinement‐induced nonlocal electromagnetic response model is applied to study radiative heat transfer processes in transdimensional plasmonic film systems. The results are compared to the standard local Drude model routinely used in plasmonics. The former predicts greater Woltersdorff length in the far‐field and larger film thicknesses at which heat transfer is dominated by surface plasmons in the near‐field, than the latter. The analysis performed suggests that the theoretical treatment and experimental data interpretation for thin and ultrathin metallic film systems must incorporate the confinement‐induced nonlocal effect in order to provide reliable results in radiative heat transfer studies. The fact that the enhanced far‐ and near‐field radiative heat transfer occurs for much thicker films than the standard Drude model predicts is crucial for thermal management applications with thin and ultrathin metallic films and coatings.

Improved Performance of CLM5.0 Model in Frozen Soil Simulation Over Tibetan Plateau by Implementing the Vegetation Emissivity and Gravel Hydrothermal Schemes

AbstractFrozen soil is widely distributed over the Tibetan Plateau (TP) and has significant impacts on the regional climate and ecosystem. However, the Community Land Model version 5 (CLM5.0) produces evident cold bias in the frozen soil simulation over TP. In this study, an improved vegetation emissivity scheme and a gravel hydrothermal scheme have been implemented into CLM5.0 model, and their synergistic influences on the frozen soil simulation over TP have been systematically addressed and revealed. Results show that adopting the vegetation emissivity scheme, the gravel hydrothermal scheme, and both can remarkably reduce the cold bias in the frozen soil simulated by the original CLM5.0 model, and the column mean root‐mean‐square error (RMSE) can be reduced by 12.88%, 20.68%, and 31.11% at Arou site and 25.03%, 10.15%, and 36.87% at Maqu site, respectively. The reductions of column mean RMSE for the modeled soil temperature regionally averaged over TP induced by the adoption of vegetation emissivity scheme, the gravel hydrothermal scheme, and both are 32.34%, 6.75%, and 30.18%, respectively. The underlying physical mechanisms are related to the improved representation of the soil hydrothermal processes above or in the soil. The improved vegetation emissivity scheme improves the soil surface long‐wave radiation heat transfer process, while the gravel hydrothermal scheme improves the hydrothermal properties within the soil. Overall, improving the surface long‐wave radiation heat transfer and the internal soil hydrothermal properties can obviously enhance the performance of CLM5.0 model in simulating the soil freeze–thaw processes.

Thermodynamic Irreversibility Analysis of Thermal Radiation in Coal-Fired Furnace: Effect of Coal Ash Deposits.

In this paper, a three-dimensional (3-D) high-temperature furnace filled with a gas-solid medium was investigated, and the radiative transfer equation and the radiative entropy transfer equation in the chamber were applied in order to analyze the effect of coal deposits on thermal radiation. The heat flux on the walls of the furnace and the entropy generation rate were determined due to the irreversibility of the radiative heat transfer process in the furnace. Furthermore, the effect of ash deposits on the wall surface on the irreversibility of the radiation heat transfer process was investigated. The numerical results show that when burning bituminous and sub-bituminous coal, ash deposits in the furnace led to a 48.2% and 63.2% decrease in wall radiative heat flux and a 9.1% and 12.4% decrease in the radiative entropy rate, respectively. The ash deposits also led to an increase in the entropy generation number and a decrease in the thermodynamic efficiency of the radiative heat transfer process in the furnace.

Thermodynamics second-law analysis in an unsteady conduction and radiation heat transfer system with internal heat source

This paper presents an improved thermodynamic analysis method used to evaluate the irreversibility and efficiency of a coupled conductive-radiative heat transfer process. The exergy analysis of the conductive-radiative process is developed based on the exergy balance equation. In the method, the local entropy generation rates due to the radiative heat transfer and heat conduction processes are calculated. Based on the entropy generation analysis, an exergy analysis is performed, the stored exergy of the system and the exergetic efficiency are determined, and the thermodynamic efficiency of the system is further evaluated. The results are verified by the principle of exergy balance and the exergy generation rate calculated by the method does not deviate from the theoretical value by more than 0.1%. Four heat source distributions are specified in the numerical analysis, which include completely uniform, partially uniform, and non-uniform heat source distributions in the numerical analysis. The effects of heat source distribution, conductive-radiative coefficient and total heat generation rate of medium on entropy generation and stored exergy of system are investigated. The numerical results show that the average temperatures and exergy storage rate of the systems with partially uniform heat source distribution are 5.1–79.7 K and 6.8–49.8% higher than the other distributions, and uniform and centralized heat generation in the internal region of the system is proposed. The total stored exergy of the systems has a tendency to first increase and then decrease, reaching its maximum at N = 1 as the conduction and radiation coefficients increase, and increasing as the total heat generation rate increases.

Determination of heat transfer mechanisms during vacuum drying of solid-moist and liquid-viscous materials

Most drying methods combine convective, conductive and radiation heat transfer processes. The share of each type of heat transfer may vary depending on the type and mode of drying, type of product, etc. In this study, the problem of determining the heat transfer mechanism of vacuum drying of solid-moist and liquid-viscous materials is solved. The objects of the study are Jerusalem artichoke tubers, camel and mare milk. The numerical values of the heat transfer components are found experimentally and their shares in the total heat flux are determined. During vacuum drying of Jerusalem artichoke at a medium pressure of 4 kPa and a temperature of 55 °C (with a layer height of 0.01 and 0.02 m), the convective component predominates (58.55 and 67.65 %). The share of thermal conduction (18.96 and 29.39 %) and radiation (13.39 and 12.05 %) is much lower. The mechanism of thermal conduction begins to prevail with an increase in the height of the material layer (0.03 and 0.04 m). The convective component is also dominant for vacuum drying of milk: at medium pressures of 6÷10 kPa and a temperature of 40 °C, its value for mare milk reaches 78.21 %, for camel milk – 73.33 %. The second most important is the share of radiation (19.45 and 22.58 %). Conductive heat transfer has the minimum indicators (5.66 and 6.17 %). The large values of the share of thermal conduction during drying of Jerusalem artichoke compared to milk are explained by the fact that heat transfer occurs inside the tubers due to conduction, and inside milk – due to convection. Insignificant shares of radiation are explained by low and medium vacuum values in the chamber. In the studied range, heat and mass transfer occurs due to molecular diffusion and convection. The results obtained can be used to formulate criterion heat transfer equations, in engineering calculations, and optimization of the vacuum dryer operation.

Comparative analysis of heat loss performance of flat plate solar collectors at different altitudes

Flat plate solar collectors (FPSC) are a vital component in the medium and low temperature utilization system of solar energy and are widely used at different altitudes throughout China. Changes in altitude can cause changes in outdoor meteorological conditions, which can affect radiation and convection heat transfer processes on the surface of FPSC. Research into the heat loss characteristics of FPSC, when used at different altitudes, is relatively inadequate. To analyze the comprehensive impact of the change in altitude on the heat loss performance of FPSC, a model for the calculation of heat losses of FPSC was developed in this study. The influence of air pressure on air convection and long-wave radiation heat transfer was considered, and the accuracy of the model was verified by experimental test data. The variation of convection heat loss (CHL) and radiation heat loss (RHL) caused by environmental conditions such as atmospheric pressure, air density, solar irradiance and water vapor partial pressure with changes in altitude is investigated using the numerical calculation method. The results show that with increasing altitude, CHL and RHL show opposite trends, with the former gradually decreasing and the latter gradually increasing. The maximum variation of CHL and RHLs as a percentage of total heat loss can exceed 50%. Two opposite processes of heat loss play a combined role in the variation of the total heat loss, resulting in a decreasing and then an increasing trend in the altitude of the total heat loss coefficient, with a maximum variation of 10%.

Towards the dynamics of a radiative-reactive magnetized viscoelastic nanofluid involving gyrotactic microorganisms and flowing over a vertical stretching sheet under multiple convective and stratification constraints

Due to the impressive industrial applications and continuous progress in the bio-nano-technological sector, the survey of MHD bioconvective flows in non-Newtonian nanofluids become nowadays the main subject of interest as this topic requires a wide range of physical importance in biofuel, bioengineering, and biomedical investigations. Motivated by the aforesaid applications, the present examination has been accomplished numerically to explore the interesting aspects of two-dimensional Walters-B nanofluid flows past a vertical elongating sheet, in which motile microorganisms swim within the medium. Herein, Buongiorno's nanofluid model has been extended realistically by adopting the modified version of Fourier’s and Fick's theories to evidence the significant roles of the haphazard motion/thermo-migration of the solid nanoparticles, the bioconvection phenomenon, the radiative heat transfer processes, and the activation energy on the flow problem under consideration. Further, the monitoring conservation equations were altered into a set of simplified differential equations via various mathematical transformations. By applying special differential quadrature schemes, the obtained nonlinear problem was tackled numerically in MATLAB software. Dynamically, it was proved that the viscoelastic parameter has a slow-down effect on the nanofluid motion, but its velocity can be speeded up enormously by upsurging the velocity ratio parameter. As other findings, it was found that the involved Biot numbers exhibit an enhancing effect on the sketches of the nanofluid temperature, the nanoparticles’ concentration, and the motile microorganisms’ concentration, whilst a reverse trend was revealed for the relaxation parameters against the thermal and nanoparticles’ concentration profiles.

Experimental investigation on the thermal performance of an enhanced convection-radiant heating wall panel with orifices

With the increase of the application scale of renewable energy low-temperature heating system, it is becoming an inevitable requirement to design a new-type heating terminal with good thermal performance, flexible adjustment, and high combination with buildings. In this study, an enhanced convection-radiant wall panel with orifices (CRWPO) is designed and tested. A full-scale CRWPO experimental system was built in a laboratory, and the radiation and convection heat transfer process were measured under different supply water temperatures, orifice diameters, and densities conditions to determine optimal operating conditions. The main intentions of the experimental research include: (1) testing the actual heating effect of the CRWPO heating terminal under the experimental conditions, (2) analyzing the impacts of different design and operation parameters on the thermal performance of CRWPO, and obtaining the ratio of convection to radiation heat dissipation, and (3) summarizing the radiant/convective heat supply of the CRWPO under different working conditions and providing a reference for the heating terminal design. The results showed that enhanced convection heat transfer is achieved by setting orifices on the radiant wall panel. Compared to a wall panel without orifices, the total heat transfer quantity increased by 57%. Furthermore, the heat flux stability time of the CRWPO is about 1–1.5 h, which is about 2–4 h shorter than that of a traditional heavy-weight radiation terminal. Furthermore, The radiation-convection heat transfer ratio decreased from 1.89 to 0.75 when the orifice diameter increased from 0 mm to 20 mm.

Disquisitions Relating to Principles of Thermodynamic Equilibrium in Climate Modelling.

We revisit the fundamental principles of thermodynamic equilibrium in relation to heat transfer processes within the Earth’s atmosphere. A knowledge of equilibrium states at ambient temperatures (T) and pressures (p) and deviations for these p-T states due to various transport ‘forces’ and flux events give rise to gradients (dT/dz) and (dp/dz) of height z throughout the atmosphere. Fluctuations about these troposphere averages determine weather and climates. Concentric and time-span average values <T> (z, Δt)) and its gradients known as the lapse rate = d < T(z) >/dz have hitherto been assumed in climate models to be determined by a closed, reversible, and adiabatic expansion process against the constant gravitational force of acceleration (g). Thermodynamics tells us nothing about the process mechanisms, but adiabatic-expansion hypothesis is deemed in climate computer models to be convection rather than conduction or radiation. This prevailing climate modelling hypothesis violates the 2nd law of thermodynamics. This idealized hypothetical process cannot be the causal explanation of the experimentally observed mean lapse rate (approx.−6.5 K/km) in the troposphere. Rather, the troposphere lapse rate is primarily determined by the radiation heat-transfer processes between black-body or IR emissivity and IR and sunlight absorption. When the effect of transducer gases (H2O and CO2) is added to the Earth’s emission radiation balance in a 1D-2level primitive model, a linear lapse rate is obtained. This rigorous result for a perturbing cooling effect of transducer (‘greenhouse’) gases on an otherwise sunlight-transducer gas-free troposphere has profound implications. One corollary is the conclusion that increasing the concentration of an existing weak transducer, i.e., CO2, could only have a net cooling effect, if any, on the concentric average <T> (z = 0) at sea level and lower troposphere (z < 1 km). A more plausible explanation of global warming is the enthalpy emission ’footprint’ of all fuels, including nuclear.

Extension of the Balbi fire spread model to include the field scale conditions of shrubland fires

The ‘Balbi model’ is a simplified rate of fire spread model aimed at providing computationally fast and accurate simulations of fire propagation that can be used by fire managers under operational conditions. This model describes the steady-state spread rate of surface fires by accounting for both radiation and convection heat transfer processes. In the present work the original Balbi model developed for laboratory conditions is improved by addressing specificities of outdoor fires, such as fuel complexes with a mix of live and dead materials, a larger scale and an open environment. The model is calibrated against a small training dataset (n = 25) of shrubland fires conducted in Turkey. A sensitivity analysis of model output is presented and its predictive capacity against a larger independent dataset of experimental fires in shrubland fuels from different regions of the world (Europe, Australia, New Zealand and South Africa) is tested. A comparison with older versions of the model and a generic empirical model is also conducted with encouraging results. The improved model remains physics-based, faster than real time and fully predictive.

New WSGG model for gas mixtures of H2O, CO2, and CO in typical coal gasifier conditions

This study develops a new weighted-sum-of-gray-gases (WSGG) model for the radiative characteristics of CO, H2O, and CO2 mixtures in pulverized coal gasification plants under a typical high pressure (45 atm). This is significant for prediction of radiative heat transfer in computational fluid dynamic (CFD) calculation. The new high-pressure WSGG model and its parameters are based on the statistical narrow band (SNB) model of EM2C Laboratory. The new model applies to the temperature range of 500–3000 K and the path length range of 0.1–20 m. Moreover, the model can calculate the radiation characteristics of gas mixtures under different partial pressures of three gases. Therefore, the new model can be applied to the radiation transfer in pulverized coal gasification under specific high pressure. The results of one-dimensional cases show that the new model can well predict the radiative heat transfer process of gas mixtures. The error of radiation heat flux is less than 5.38%, and the radiation source term error is less than 0.67%, while the accuracy of the traditional WSGG model is very low under gasification conditions, and the error is more than 25% compared with the benchmark. When the new model is embedded into the CFD simulation of a high-pressure gasifier, the maximum temperature difference between the new and traditional models is up to 300 K, while the calculated absorption coefficient and radiation source term of gas mixtures are noticeably different. In conclusion, compared with the traditional model, the new WSGG model has higher accuracy for calculating gas radiation heat transfer under the condition of coal gasification. This paper also provides the parameters of the new model, which can be used in the CFD engineering simulation of coal gasification plants.

Passive daytime radiative cooling: Fundamentals, material designs, and applications

AbstractPassive daytime radiative cooling (PDRC) dissipates terrestrial heat to the extremely cold outer space without using any energy input or producing pollution. It has the potential to simultaneously alleviate the two major problems of energy crisis and global warming. In this review, we summarize general strategies implemented for achieving PDRC and various applications of PDRC technologies. We first introduce heat transfer processes involved in PDRC, including radiative and nonradiative heat transfer processes, to evaluate the PDRC performance. Subsequently, we summarize the general material designs used for controlling PDRC performance, such as tuning the thermal mid‐infrared emittance and solar reflectance. Finally, we discuss the diverse applications of PDRC technologies to overcome problems in space cooling, solar cell cooling, water harvesting, and electricity generation.image

Radiative Heat Transfer Processes in Heating Open Platforms

The results of a numerical analysis of the parameters of the system of radiative heating of an open platform, a flat car of size 10 × 75 m, are presented. The temperature of the radiator of size 0.3 × 9 m was 600–900 K. The temperature of the atmospheric air varied in the range from –10 to +10°C at a wind velocity of up to 5 m/s. As the criterion of heating comfortably a human being, the temperature at the level of his head (175 cm) was taken. As a result of the computational experiment, the dependences of the temperature of the heated platform on the temperature and the height of the radiator mounting, as well as on the direction and velocity of wind, were obtained. It has been established that the temperature at the level of the top of the human being’s head is equal from 2–3 to 16–18°C when the radiators are disposed at a height of 4–8 m from the platform surface at the surrounding air temperature of 0°C and wind velocity of 0.5–5 m/s. It is shown that on decrease of the air temperature to –10°C the installation of wind protection is required.