Unruled Notebook

Here is the rambunctious boiling song from the one and only kitchen band. [Boiling Song MP3 File. Size is 1.1 MB, links to boxnet page for play and download. See Note 2 for copyright]. Listen to the song before you read this essay. The song is a mix of hard rock and grunge instrumental with high-pitched multiple wailing guitar sounds and cacophonous tempo changing beats. But I assure you, no musical instrument has been used for recording this song.

The sound is really cacophonous (what else you expect from a hard rock?). Take precautions like wearing a headphone. Now for some sleeve notes about the record.

The Kitchen Band comprises of Gas Stove, Water Vessel, Heat Energy and Mother Nature.

We all have witnessed the phenomenon of boiling. For most of us, it begins in our kitchen or bathroom and proceeds until the dining room and perhaps stops there, forever. For the purpose of this essay we shall take it a bit further. Boiling comes in several shapes and sounds, grouped under the two broad categories, pool boiling and flow boiling. Unlike an Italian Screwdriver (which is not a screwdriver made in Italy), Pool Boiling, as the name suggests is associated with a boiling pool of liquid. It is the boiling of a stationary mass of liquid. An example is the familiar water boiling in the pan on a kitchen stove. Flow boiling is means flow of the boiling fluid. An example is the flow inside the boiler tube (water wall panels) of a thermal power plant.

Pool boiling includes five regimes of reasonably distinct characterizations. Taking the familiar liquid water as the example liquid, let us start heating it in a kitchen vessel kept over our standard gas stove to explore. Along the way, we shall create the song.

As we know, water boils at 100 degree C. Not exactly true. A better way to say that is liquid water that is in contact with its vapour boils at 100 degree C. If it is not in contact with its vapour, but still in contact with a heating surface, it continues to remain in liquid form and raises its temperature to more than a few degrees above 100 degree C, for conventional machined surfaces like our kitchen vessel. Then it bubbles up. This range of heating of water from its room temperature falls within a regime that is identified with natural convection of water [1]. See how the red curve is depicted in the left bottom corner of Fig. 1 below.

An explanation of Fig. 1 is now in order. The abscissa is the excess temperature between the wall temperature (the bottom surface of the kitchen vessel kept over the stove) and the saturation temperature of the fluid (in our case, water). The ordinate is the heat flux that is released into the boiling fluid (in our case, water). The red curves are paths that characterize what happens to a fluid undergoing pool boiling in all of its five distinct stages.

This region is marked by a bubble inception point where the first bubble can be noticed in our vessel of heated water. Beyond this region, water boils and the nucleate boiling phase begins. This means, initially isolated bubbles are formed in the nucleation sites, which are nothing but gaps or imperfections in the heater surface – in our case, the bottom inner surface of the heated vessel on the stove. When the nucleation sites become aplenty, as the heating increases (see Fig. 1), the bubbles generated from these sites merge together to form vertical columns and slugs that could in principle reach the top free surface of water in the vessel.

No commercial kitchen stove is capable of supplying heat energy fast enough to the water to reach even the slugs and columns regime. So we may not see this range in our kitchen experiment, unless we conjure up a mini nuclear reactor as our kitchen stove. In which case, keep me informed of it, before you invite me over for dinner.

Proceeding to heat and boil the water beyond the slugs and columns regime in a laboratory set-up, there results a situation when a peak heat flux value is reached (marked q_max^,, in the ordinate of Fig. 1). Further increase of heating results in sudden increase of temperature difference (marked in the abscissa). This mostly leads to the melting of commonly used heater material surface. The transition boiling region connects this peak heat flux limit with the film boiling regime, wherein the heater surface is completely blanketed by a film of vapour of the liquid (water, in our case). Heat transfers across the film, into the liquid water residing unstably above the vapour layer.

Further explanations about many of these interesting phenomena are kept out of this essay. There are separate monographs of knowledge available for pool boiling alone (and more for flow boiling). Take a look at the reference [1]. We now go back to the song of our kitchen band.

We now understand the simple kitchen experiment we have performed. In this experiment, just after the bubble inception point, hot vapour bubbles form near the bottom of the vessel (close to the stove) and raise to the top surface of the vessel through the liquid water. The rest of the water column along the vertical path of the bubble is colder than the hot vapour bubble. So while raising, when the top surface of these bubbles come in contact with the colder water above, collapses suddenly by condensing. This cavitation collapse of the vapour bubble results in the high pitched noise (ping) that falls within our audible range.

The noise is a mixture of pings from several thousands of bubble collapse at the same instant. The process of bubble formation and collapse at a higher point is repeated at a high frequency resulting in us hearing the high pitched sound continuously. Further, the smaller the bubble size, the higher the pitch. Longer the vertical column of water, longer the duration of singing one can hear. This is because the vertical residence distance (hence the cooling distance) for the bubbles is increased before they reach the top free surface of the water inside the vessel.

At an instance, bubbles of several sizes are always present in the boiling process. The singing we hear is always a mixture of sounds of several frequencies influenced further by the cross dispersion before it reaches our ears. See for instance, in Fig. 2, the bubble distribution across the surface of the water inside the kitchen vessel.

[The above picture is taken in our kitchen and doctored a bit to visualize the bubbles better. Sophisticated doctoring techniques exist to better capture these bubbles.]

As the heating results in further increase of temperature, the bubble sizes also grow bigger and the sound becomes muted. The bubbles no longer condense within the water column in the vessel but reach the surface of the water and escape the vessel. Water in the vessel continues to boil with a dull gurgling sound. We don’t hear this in our recorded song as it takes a while in my experiment to reach this stage (about 4 minutes for the vessel I used). I have cut those parts out. The outro that one hears in the end part of the audio is the well isolated bubble situation, when the system cooled down after the stove is switched off.

The staccato drumming that one hears over the singing (the wailing guitars like sound) is more due to the pinging noise from the collapse of the bubbles that originally form on the inner surface of the hot side walls of the stainless steel vessel that is used in the experiment. One could reduce this drumming noise with another type of vessel. But the initial singing depends on the nucleation sites at the bottom of the vessel. The required size of these nucleation sites range between 0.005 mm to 5 micro-m [1] – a range in tune with the commercially manufactured surfaces (of cooking vessels).

All the noise explained so far and the accompanied phenomena are well within the bubble inception and isolated bubble region of Fig. 1. We don’t reach the slugs and columns range of Fig. 1 in our kitchen. There ends the sleeve notes for the boiling song from our kitchen band.

Wouldn’t you want to go back now and enjoy listening to the nucleate boiling bubble cavitation grunge again?

Notes

[1] The explanation given in this essay is not rigorous in many places. The essay should serve as only a starting point for understanding the phenomena of boiling. Consult text books (one is given in [1]) for quantitative explanations.

[2] The boiling song audio given in this essay is copy-lefted. It is available for copy-lifting under a re-creative non-commons license. For hearing a live version of it with a variation of the theme, next time stay close to the hot water that you are making.

Reference

[1] A Heat Transfer Text Book by John H. Lienhard IV and John H. Lienhard V. Website for download: http://web.mit.edu/lienhard/www/ahtt.html