Asking this because this came up in a video game I was playing, where a character was telling a story of how he had seen enemies burned as an offering to Sutr, the fire giant. The game is Senua's sacrifice.
Did these sacrifices happen to appease the gods?
While it is a bit tricky, I'm sure that the Vikings did not perform the human sacrifice at least as OP asked above, tl;dr:
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1: Burning as a Form of Human Sacrifice in Northern Europe during the Viking Age
The following passage is taken from English translation of the Chronicle of Thietmar of Merseburg (I-17) that I regard as rather (at least relatively) more trustworthy than famous (notorious) description of the ritual held in Gamla Uppsala by Adam of Bremen:
'.....In those parts, the center of the kingdom [of the Danes] is a place called as Lejre, in the region of Seeland. Every nine years, in the month of January, after the day on which we celebrate the appearance of the Lord [6 January], they all convene here and offer their gods a burnt offering of ninety-nine human beings and as many horses, along with dogs and cocks - the later being used in place of hawks. As I said, they were convinced that they would do service for them with those who dwell beneath the earth and ensure their forgiveness for any misdeeds. Our King [Henry I of Germany] did well when he forbade them to practise [sic] such an execrable rite......' (Warner (trans.) 2001: 80).
If we believe Thietmar (most of the researchers don't take all of his description at face value, though), this offering was performed in Denmark in the first half of the 10th century. It is also worth noting that he specifies the burning as a form of offering to the super natural beings in general, not to a unique deity as Surt the Fire Giant.
Scholars of older generations had tried hard to find the parallel between the form of offering and the trait of the deity that Norse people offered to, i.e. the ritual hangings as a special offering sacrifice to Óðinn, but they have recently got more pessimistic to establish this kind of exact correlation (Cf. Pluskowski 2000 in Ibid. (ed.) 2000). In other words, the burned human sacrifice did not necessarily means that the dead was offered to the deity of fire like Surt.
2: Surt Cult during the Viking Age?
It's well-known that Surt would play an important role in Ragnarök in Poem Völuspá (Prophecy of the Seeress), but we know really little about him except for it. If we accept the classic hypothesis of identifying him primarily as 'volcano-giant' (Cf. Lindow 2001: 283), it would be much more difficult to assign some significance to him especially in Scandinavian without any active volcano.
3: Did the Vikings sacrificed War Captives to the Gods?
It's true that the Norse people (Scandinavians) during the Viking Age performed the human sacrifice, as cited above, but AFAIK almost all of the contemporary written evidence deals with the seasonal offering (blót), and the offerings of animals were probably more important than those of humans.
Before the Viking Age, we have certainly some archaeological evidences as well as classical texts that the Scandinavians and some Germanic people used to offer the weapon of enemies as well as enemies after the battle, often deposited in lakes, wells, bogs, and on land. In course of the 6th -7th century, however, the predominant form of offering underwent some changes and only the 'booty sacrifice' of boats and weapons [after the battle] seemed to survive into the Viking Age (ca. 800-1050) as for the offerings after the battle (Patrick in Pluskowski (ed.) 2000: 22).
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