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Introduction
Figure 1. An image in the mid-infrared light spectrum of the 'Pillars of Creation", which astronomers call an incubator for new stars, captured with the MIRI optical module of the James Webb Space Telescope.
The James Webb Space Telescope is a remarkable feat of engineering, only possible due to innovations in almost all areas related to its development. The $10 billion space observatory was built to capture images of the first galaxies and stars in the universe, and extend our knowledge of the birth of stars, galaxies and even the universe to unprecedented levels (Figure 1). One of the instrument making these observations is the Mid-infrared Instrument (MIRI), which is capable of detecting light at wavelengths up to 28.5 microns. Video 1 demonstrates the path the light takes inside the instrument, from entering to reaching the optical detector which is made from Arsenic-doped silicon. ENGF0004 P a g e 5 | 12
Video 1. Video of the light path inside the MIRI instrument before it reaches the detector. Click on the video to be directed to YouTube, where you can watch it in full (1 minute watch). Source: ESA
Because all colder objects (room temperature and below) glow with infrared light due to their heat, MIRI is especially sensitive to thermal noise, or in other words, disruption due to the heat of its detector and surrounding parts. For this reason, it needs to be kept exceptionally cold at temperatures below 7 K by means of a cooling system. This is delivered through a cryocooler, which is itself remarkably innovative, relying on thermoacoustics and the cooling of gases upon adiabatic expansion (the Joule-Thompson effect) to obtain this level of cooling. In this coursework, you will explore different heat transfer processes inside MIRI, developing different mathematical models to study them and discuss the implications of your findings.
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Model 1: Heat conduction inside the MIRI detector [90 marks]
The MIRI detector is designed to detect infrared light up to 28.5 m in wavelength. The relationship between its maximum operating temperature ( measured in degrees Kelvin, K) and the maximum detectable wavelength (measured in m) is governed by the equation = 200
. . (1) To guarantee this temperature is maintained, the detector is cooled by the cooler system described in Figure 2. The MIRI detector is made of arsenic-doped silicon, and its depth is 35 mm. The one-dimensional heat equation
=
2 2 . (2) can be used to model the heat conduction through the depth of the MIRI detector, where (, ) is the temperature in K and which is dependent on both space and time, is the thermal conductivity of the material ( W m.K ), is its specific heat capacity ( J kg.K ), and is the density of the material (kg/m3). The constant terms
can be combined into a single coefficient
= , called the diffusivity constant. Your analysis in the first model, guided through the three questions, will be aimed at predicting at what time, following the start of maximum cooling, MIRI reaches operating temperature. Figure 2. Cooler system of the MIRI instrument. Two shields provide passive cooling to ambient temperatures of 40K and 20K respectively. The two-stage cryocooler ensures the detector is kept at operating temperatures, by means of refrigeration with Helium gas cooled to 18K during the first stage and to 6K during the second stage. Stage 1: Thermoacoustic cryocooler Secondary Heat Shield, ambient temperature: 20K Stage 2: Joule -Thompson loop in cryocooler Helium gas MIRI detector 17K 6K Primary Heat Shield, ambient temperature: 40K ENGF0004 P a g e 7 | 12
Question 1 [10 marks] As a starting point in analysing the temperature variation in the MIRI detector (a diagram for which is provided in Figure 3), a number of simplifying initial and boundary conditions can be applied • Initially, the detector’s temperature is determined by the passive cooling provided by the secondary shield, • the side which interfaces with the cooling liquid is kept at 6K, • through the detecting side of the detector no heat transfer occurs.
a) [5 marks] Research the literature to determine the coefficients describing the thermal properties of silicon: and , its density , and the resulting diffusivity constant . Remember to include units and your sources. b) [5 marks] Write the initial and boundary conditions described above in mathematical language.
Figure 3. Diagram of the cross-section of the MIRI detector.
Question 2 [55 marks] If boundary conditions are equal to zero when using the separation of variables method to solve PDEs analytically, the process of determining coefficient values in the general solution is simplified. In order to make use of this, it is often convenient to perform a variable transformation in which the dependent variable is represented by two parts: a steady-state part , and a transient part , = + . The conditions described in Question 1 would lead to a steady-state solution of constant 6K for the temperature ( = 6K) throughout the detector. a) [10 marks] Derive the one-dimensional heat equation and initial and boundary conditions in terms of the transient temperature , using the two-part representation of the temperature as a starting point. ENGF0004 P a g e 8 | 12
b) [10 marks] Apply the separation of variables method to split the solution for in a time and space-dependent component, and obtain the two resulting ODEs. c) [20 marks] Solve analytically these ODEs and obtain a solution for the transient temperature and from there for the temperature . d) [15 marks] Implement the obtained solution for in MATLAB and use graphs to report the time when the entire detector has reached operating temperature levels (below the as defined by Eq. (1)). Discuss briefly the behaviour of the solution as time progresses.
Question 3 [25 marks] The assumptions for the boundary conditions so far have been simplified to allow the analytical study of the system’s behaviour. If we employ a numerical solution scheme, we may be able to find solutions with more varied initial and boundary conditions such as accounting for internal heat generation inside the detector, for example, as it is hit by photons. As a first step when implementing a numerical solution scheme, it is important to validate its accuracy by comparing its solution to a solution of a simplified problem which can be solved analytically. This is your task in this question. a) [25 marks] Set up a numerical solution scheme for the heat equation and solve Eq.(1) for , given the initial and boundary conditions prescribed in Question 1. Validate the accuracy of this solution for your chosen size of the space-step and time-step, by comparing it to the analytical solution you obtained in Question 2.
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Model 2: Heat exchanger model [90 marks] Now that you have explored the temperature variation in the detector, it is useful to study the system which delivers this cooling itself. The cryocooler is made up of three consecutive heat exchangers, bringing the temperature of the helium gas running through it from 300K to 17K. A final Joule-Thompson loop can be activated to provide maximum cooling to of the Helium to 6K. Since this is a very complex system in its entirety, you will focus your analysis on a small part of the whole: the heat transfer and temperature change of the one half of the first heat exchanger. To simplify the analysis of this process, it is possible to represent it through an equivalent electric circuit, as shown in Figure 4. This approach is common when modelling heat processes and is similar to the analogy between second order spring-mass-damper mechanical systems and RLC (resistor-inductor-capacitor) electrical circuits. This method of finding simpler, well-studied equivalent models is common and very useful in engineering practice.