Preterm infants face difficulties in maintaining their body temperature due to low metabolic rates of heat generation. Therefore, using incubators and radiant warmers is crucial for their comfort and health. The objective of the present work is to analyze the heat transfer processes for a preterm infant nursed in a simplified incubator under two different operating conditions: in the first one a classical simple incubator is considered and in the second case radiant heaters are used. This is achieved by using ANSYS Fluent 19.0 which his based on the finite volume method to discretize the Navier-Stokes, energy and radiation transfer equations.  Conduction, convection and radiation heat transfer modes are modelled in the simulations and coupled to empirical correlations for metabolic heat generation and evaporative heat losses. The different incubator scenarios considered here are compared in terms of convection and radiation heat losses and skin temperature to access the thermal comfort of the preterm infant

Habchi, Charbel

Hannouch, Aziza

Khoury, Khalil

Lemenand, Thierry

English

International audiencePreterm infants face difficulties in maintaining their body temperature due to low metabolic rates of heat generation. Therefore, using incubators and radiant warmers is crucial for their comfort and health. The objective of the present work is to analyze the heat transfer processes for a preterm infant nursed in a simplified incubator under two different operating conditions: in the first one a classical simple incubator is considered and in the second case radiant heaters are used. This is achieved by using ANSYS Fluent 19.0 which his based on the finite volume method to discretize the Navier-Stokes, energy and radiation transfer equations. Conduction, convection and radiation heat transfer modes are modelled in the simulations and coupled to empirical correlations for metabolic heat generation and evaporative heat losses. The different incubator scenarios considered here are compared in terms of convection and radiation heat losses and skin temperature to access the thermal comfort of the preterm infant

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Coupled radiative and convective heat losses from preterm infant inside an incubator with radiant heaters

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CONTRIBUTED SESSIONSBio-medicineCoupled Radiative and Convective Heat Losses from Preterm Infant Inside an Incubator with Radiant HeatersA. Hannouch, T. Lemenand, K. Khoury and C. HabchiVIII International Conference on Computational Methods for Coupled Problems in Science and Engineering COUPLED PROBLEMS 2019 E. Oñate, M. Papadrakakis and B. Schrefler (Eds)    COUPLED RADIATIVE AND CONVECTIVE HEAT LOSSES FROM PRETERM INFANT INSIDE AN INCUBATOR WITH RADIANT HEATERS AZIZA HANNOUCH*, THIERRY LEMENAND*, KHALIL KHOURY† AND CHARBEL HABCHI‡ *LARIS, EA 7315, Polytech Angers, 49000 Angers, France  e-mail: aziza.hannouch@etud.univ-angers.fr, thierry.lemenand@univ-angers.fr web page: http://laris.univ-angers.fr  †Lebanese University, Faculty of Engineering, 1003 Hadath, Beirut, Lebanon Email: kekmam@gmail.com web page: https://www.ul.edu.lb  ‡Notre Dame University - Louaize, Thermofluids Research Group, 1200 Zouk Mosbeh, Lebanon Email: charbel.habchi@ndu.edu.lb web page: http://www.ndu.edu.lb  Key words: Preterm neonate, Infant incubator, Convection heat transfer, Radiation heat transfer, Thermal comfort. Abstract. Preterm infants face difficulties in maintaining their body temperature due to low metabolic rates of heat generation. Therefore, using incubators and radiant warmers is crucial for their comfort and health. The objective of the present work is to analyze the heat transfer processes for a preterm infant nursed in a simplified incubator under two different operating conditions: in the first one a classical simple incubator is considered and in the second case radiant heaters are used. This is achieved by using ANSYS Fluent 19.0 which his based on the finite volume method to discretize the Navier-Stokes, energy and radiation transfer equations.  Conduction, convection and radiation heat transfer modes are modelled in the simulations and coupled to empirical correlations for metabolic heat generation and evaporative heat losses. The different incubator scenarios considered here are compared in terms of convection and radiation heat losses and skin temperature to access the thermal comfort of the preterm infant.    1 INTRODUCTION In 2010, 11.1% of babies were born premature [1]. These babies face difficulties keeping a controlled body temperature without external assistance. Consequently, they can suffer from cold stress and hypothermia. Therefore, it is crucial to maintain an optimal thermal and humidity environment of the neonate by using incubators. Infant incubators are essential tools to provide thermal comfort for neonates especially 342Aziza Hannouch, Thierry Lemenand, Khalil Khoury and Charbel Habchi.  2 premature and sick infants. The complex heat transfer process in these systems combines convection, conduction, thermal radiation and evaporation. Incubators are used since 1860 in order to maintain an optimal thermal environment for the neonate [2]. Several studies were conducted in the open literature to evaluate the dry and latent heat losses from neonates nursed inside incubators. These studies are classified in three main categories, theoretical bioheat modeling, numerical and experimental. Bioheat modeling and thermoregulation are based on mathematical models in order to analyze the bioheat transfer in the neonate body and to determine its thermal responses to ambient conditions [3–5]. Numerical simulations of heat losses from neonate use the computational fluid dynamics (CFD) method. The aim of this method is to optimize existing devices and to design new techniques aiming to enhance the thermal comfort of neonates inside the incubators [6–8].  Experimental study are performed on cohort of human neonates or thermal manikins in order to evaluate convection, radiation and evaporation heat transfer coefficients and thermal balance in preterm neonates [9–12] The aim of the present paper is to perform numerical simulations of the coupled convective and radiative heat transfer in infant incubators under two different conditions. The first case the incubator walls are at mean radiant temperature while in the second case, the incubator walls are assumed to radiate heat at a given heat flux so that to reduce radiation heat losses from the infant skin.  2 COMPUTATIONAL DOMAIN AND BOUNDARY CONDITIONS An anthropomorphic 3D model representing a preterm neonate is used in the present study. Its dimensions correspond to a preterm with 35 week of gestational age in 50th percentile having a mass of 2.3 kg [13,14]. The neonate is shown in Figure 1 and it has a surface area of 0.133 m2 and a body length of 45 cm. The neonate 3D model was built using the SolidWorks CAD software and imported to ANSYS Space Claim in which further modifications are performed. The infant is then inserted in a simplified incubator 25 cm in width, 20 cm in height and 60 cm in length. The incubator has one inlet and one outlet both located at the mattress level as shown in the figure below. The air stream entering the incubator has a speed of 0.2 m/s and a temperature of 38℃. The outlet is assumed at atmospheric pressure. No slip boundary conditions are imposed on all solid surfaces. To mimic metabolic heat generation, a constant heat flow is imposed on the outer surface of the model according to the empirical equation obtained by Bruck [15] which depends on the infant mass 𝑚𝑚𝑖𝑖𝑖𝑖𝑖𝑖 and age in days 𝑡𝑡𝑖𝑖𝑖𝑖𝑖𝑖 (Eq. (1)). Therefore, for the present simulations the surface heat flow is fixed to 4.23 W. 𝑞𝑞𝑚𝑚 = 𝑚𝑚𝑖𝑖𝑖𝑖𝑖𝑖(0.0522𝑡𝑡𝑖𝑖𝑖𝑖𝑖𝑖 + 1.64) (1) The thermal emissivity of the neonate surface is assumed 0.95 [10] and the mean radiant temperature of surrounding surfaces is obtained from empirical correlation obtained by Decima et al. [16] and depending on the air temperature 𝑇𝑇𝑎𝑎 as given below: 𝑇𝑇𝑟𝑟 = 0.724(𝑇𝑇𝑎𝑎 − 31.93) + 29 (2) Moreover, evaporative heat loss from the skin are obtained from Lyon and Oxley [17] and it depends on the surface area of the neonate and humidity inside the incubator which was 343Aziza Hannouch, Thierry Lemenand, Khalil Khoury and Charbel Habchi.  3 assumed equal 70%. Thus the resulting evaporative cooling is 1.25 W.  Figure 1: Isometric view of the 3D model representing the preterm neonate nursed inside a simplified incubator Two different cases are studied in this paper. In the first one all surfaces surrounding the baby (mattress and incubator walls) are assumed at standard radiant temperature obtained from Eq. (2). In the second case, radiant heating elements are used and thus the incubator walls are now assumed to radiate heat at an average heat flux of 0.55 W/m2. 3 NUMERICAL METHOD A non-uniform polyhedral mesh is used in the present study with 4 prism layers refined near the neonate skin surface. The total number of elements is around 840,000 cells. A mesh sensitivity analysis was conducted prior to choosing this mesh density in order to verify mesh convergence. The criteria for the mesh convergence are the surface temperature and total surface heat flux. Navier-Stoke and energy equations for laminar flow are using ANSYS Fluent 19.0 which based on the cell centered finite volume method. Second order double precision numerical schemes are used for diffusive and convective terms and Coupled model is adopted for velocity-pressure coupling. The buoyancy effects are included by using the Boussinesq approximation for air density. Radiation heat transfer is computed using the Discrete Ordinates Method which solves the radiative transfer equation for a finite number of discrete solid angles. The energy equation for heat losses from preterm infants is given below: 𝑞𝑞𝑚𝑚 − 𝑞𝑞𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑞𝑞𝑐𝑐𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑐𝑐𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 𝑞𝑞𝑒𝑒𝑒𝑒𝑟𝑟𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 𝑞𝑞𝑐𝑐𝑒𝑒𝑒𝑒𝑟𝑟𝑐𝑐𝑐𝑐𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 (2) The two terms on the left hand side are obtained from empirical equations as discussed previously while the terms on the right hand side are computed. For both cases studied in this paper, conduction heat transfer was very small relative to the other modes of heat losses. Thus we will focus on convective and radiative heat losses and their effects on the neonate skin temperature in the next section. 4 RESULTS Figure 1 shows the heat rates from the infant nursed inside the incubator for both studied 344Aziza Hannouch, Thierry Lemenand, Khalil Khoury and Charbel Habchi.  4 cases. The metabolic heat generation is lost through latent heat by evaporation which is the same for both cases since it was obtained using empirical correlation. Meanwhile, radiation heat losses from the neonate are reduced due to the addition of radiant heaters. These radiant heaters will lead also to a slight decrease in conduction heat losses since the mattress temperature will increase. However, convective heat losses have increased since as will be seen next, the skin temperature exceeds the surrounding air leading to higher convective heat loss. However, the overall impact is beneficial since the thermal comfort is enhanced from skin temperature point of view.  Figure 2: Heat losses from the infant skin for both studied cases The skin temperature distribution is shown in Figure 3 for both cases. For the case without radiant heaters, the average skin temperature is around 35℃ which is smaller than the comfort temperature at 37℃. The temperature is increased by addition of radiant heaters and its average value reaches around 37℃ leading to better thermal comfort when compared to the reference case without radiant heaters.  345Aziza Hannouch, Thierry Lemenand, Khalil Khoury and Charbel Habchi.  5   (a) (b) Figure 3: Skin surface temperature (a) without radiant heaters and (b) with radiant heaters 5 CONCLUSION In this study, convective and radiative heat losses from preterm infant nursed inside an incubator are studied under two different conditions. In the first case, a simplified classical incubator is used, while, in the second case radiant heaters are added. The incubator walls are modeled thus as radiant heaters with imposed heat flux. The flow and heat transfer are computed using ANSYS Fluent 19.0 which is based on the finite volume method. A polyhedral mesh and second order numerical schemes are adopted for the simulations. The results show a decrease of radiation heat loss when adding radiant elements whit a slight increase in convective heat loss. Moreover, the average skin temperature of the neonate was increased from 35℃ to 37℃ proving the benefits of these radiant heaters. In future studies we will use experimental method for a preterm thermal manikin nursed inside a CALEO incubator to validate our numerical simulations. ACKNOWLEDGMENT This research was supported by the AUF CNRSL research grant. The authors are also grateful to Dräger and PrimeMedical for donating a Caleo infant incubator. REFERENCES [1] World Health Organization, Born Too Soon: The Global Action Report on Preterm Birth, New York, 2012. [2] S.N. Sreenath, S. Kumar, H.S. Lohit, M. Student, Design of an Infant Incubator for Cost Reduction and Improved Usability for Indian Health Care Centers, SASTECH J. 11 (2012) 82–89. [3] D. Fiala, Dynamic simulation of human heat transfer and thermal comfort, De Montfort University, 1998. [4] C.B. 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Coupled radiative and convective heat losses from preterm infant inside an incubator with radiant heaters

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