The phrase "a single flame" is correct and usable in written English.
It can be used to describe one flame, often in a metaphorical or literal context, such as in poetry or descriptive writing.
Example: "In the darkness of the night, a single flame flickered, illuminating the path ahead."
Alternatives: "one flame" or "a lone flame".
He drew a large triangle within which there was not a single flame marker.
In the deeper mines the heat can reach 400 Celsius, and men work by the light of a single flame.
When no hint of risk or chance remains, even in the leap of a single flame, where does the drama let alone the peril reside?
From that road, the first thing you see is a tall pipe with a single flame, which burns above the glare of the steelworks.
The long stems of the cauldron then gently rose towards each other and converged to form a single "Flame of Unity".
With their details suppressed through soft focus, buildings are distilled to Platonic ghosts of structures: a Gaudí interior is an insouciant twist of line; a house designed by Gerrit Rietveld, a walk-in constructivist painting; the interior of Le Corbusier's Chapel of Notre Dame du Haut, a giant lantern illuminated by a single flame.
The possible burning structures were observed to be: (1) a single-flame (a lean or rich premixed flame); (2) a double-flame (two lean or rich premixed flames, or a rich premixed flame and a diffusion flame); and (3) a triple-flame (a rich premixed flame, a diffusion flame and a lean premixed flame).
The seven young torchbearers then ignited a tiny single flame, triggering the ignition of 204 copper petals carried into the stadium by the athletes.
The representative edge propagation speed of a single lifted flame or an upstream lifted flame in the case of twin lifted flames increased as the liftoff height became higher.
Jon Henley in the Guardian ("A century to count the cost of war") chose to write about the Mons ceremony below a picture of the single flame burning by the grave of the Unknown Soldier in Westminster Abbey.
This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased.
